Photon counting and spectroscopy

ABSTRACT

A measurement system includes an optical source (e.g., laser) to irradiate a sample (e.g., a cell); a solid-state photon detector (SSPD) to receive resultant light from the sample; and a photon counter to count photons received by the SSPD. The photon counter can include a differentiator to provide a differentiated photon signal and a crossing detector configured to count photons based on a number of times the differentiated photon signal crosses a predetermined threshold level. In some examples, a pulse detector can provide a pulse-width signal from the SSPD output photon signal, and a pulse counter can count based on both a number of pulses and widths of the pulses. The SSPD can include a silicon photomultiplier (SiPM) array or a solid-state photomultiplier. Some examples use the measurement system to measure samples in fluids, e.g., in flow cytometers or multi-well plates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of a U.S. nationalizationapplication Ser. No. 16/604,557, filed Oct. 10, 2019 entitled PhotonCounting and Spectroscopy from an international application No.PCT/US18/27302, filed Apr. 12, 2018 entitled Photon Counting andSpectroscopy which claims priority to, and the benefit of, U.S.Provisional Patent Application No. 62/485,207, filed Apr. 13, 2017 andentitled “Photon Counting and Spectroscopy,” which is incorporatedherein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features, and advantages of various aspects will becomeapparent when taken in conjunction with the following description anddrawings. Identical reference numerals have been used, where possible,to designate identical features that are common to the figures. Theattached drawings are for purposes of illustration and are notnecessarily to scale.

FIG. 1 shows a block diagram of an example excited-photon spectrummeasurement/analysis system.

FIG. 2A shows an example structure of a silicon photomultiplier (SiPM)circuit.

FIG. 2B shows example characteristics of an SiPM.

FIG. 2C is an elevational cross-section of an example SiPM.

FIG. 2D shows electric-field characteristics of an example SiPM.

FIG. 3A is a graphical representation of a micrograph of an exampleSiPM.

FIG. 3B is a circuit diagram of an example SiPM.

FIG. 4A shows an example SiPM readout circuit including adifferentiator, and related SiPM components.

FIG. 4B shows measured data of an output from the circuit of FIG. 4A fora single photon pulse.

FIG. 4C shows measured data of an output from the circuit of FIG. 4A formultiple photon pulses.

FIG. 5 shows an example differentiator circuit usable, e.g., in thecircuit of FIG. 4A.

FIG. 6A shows measured data of an example input to the differentiationstage of the circuit of FIG. 5 .

FIG. 6B shows measured data of an example output from thedifferentiation stage of the circuit of FIG. 5 , for the input of FIG.6A.

FIG. 7 shows an example adaptive comparator circuit.

FIG. 8 shows example inputs and outputs of the adaptive comparatorcircuit of FIG. 7 .

FIG. 9 shows a block diagram of signal processing for photon detection,and related components.

FIG. 10A shows an example circuit configuration of an adaptive pedestalclamping circuit.

FIG. 10B shows another example circuit configuration of an adaptivepedestal clamping circuit.

FIG. 11A is a graphical representation of a micrograph of an exampleSiPM.

FIG. 11B depicts characteristics of longitudinal multiplication in anSiPM.

FIG. 11C depicts characteristics of transverse multiplication in anSiPM.

FIG. 12 shows simulated inputs and outputs of the circuit of FIG. 13 .

FIG. 13 is a circuit diagram of an example filter section in anenvelope-detector circuit.

FIG. 14 is a circuit diagram of an example pulse-widthcounter/discriminator circuit.

FIG. 15 shows example timing diagrams of the circuit of FIG. 14 .

FIG. 16 shows an example spectrofluorometer configuration according tovarious prior schemes.

FIG. 17 shows a single-photon spectrometer configuration according tovarious examples herein.

FIG. 18 shows sample-illumination and resultant-light-collection opticsaccording to various examples herein.

FIG. 19 shows some components and optical paths of an example motorizedmonochromator.

FIG. 20 shows an example configuration of an optical system permittingwall-less fluorescence detection of liquid samples by coaxialillumination in a tube.

FIG. 21A shows a spectrum spread spatially, and example fiber-bundleshapes.

FIG. 21B depicts components and operation of an example monochromator.

FIG. 21C shows a cross-section illustrating dense packing of opticalfibers.

FIG. 21D shows an example circular fiber bundle.

FIG. 21E shows another example circular fiber bundle.

FIG. 21F shows an example rectilinear fiber bundle.

FIG. 21G shows another example rectilinear fiber bundle.

FIG. 22 shows an end view of an example using a relatively large-corefiber and a slit aperture at the end of the fiber.

FIG. 23 shows components of an example optical system supporting on-axisand perpendicular measurement.

FIG. 24 shows an example flow-cytometry system according to variousexamples herein.

FIG. 25 shows an example data-processing system according to variousexamples herein.

FIG. 26A shows a perspective of components of an example motorizedmonochromator.

FIG. 26B shows an elevational cross-section of an example polychromator.

FIG. 27A shows an example linear fiber array and connector.

FIG. 27B shows output fibers fed by a polychromator.

FIG. 28 shows an example SiPM linear array.

FIG. 29 shows components of an example optical system and array.

FIG. 30 shows an example system for high-throughput sample measurement.

FIG. 31 is an isometric view of tubes in a portion of a plate array suchas that in FIG. 30 .

FIG. 32 shows a plan view of an example silicon microelectromechanicalsystem (MEMS) photomultiplier (PM).

FIG. 33 shows an elevational cross-section of the PM of FIG. 32 .

FIG. 34 shows an example photon-counting measurement system, e.g., usinga PM.

FIG. 35 shows circuitry of an example silicon photomultiplier.

FIG. 36 shows circuitry of an example silicon photomultiplier includinga fast readout electrode.

FIG. 37 shows an equivalent circuit illustrating characteristics of thephotomultiplier of FIG. 36 .

DETAILED DESCRIPTION

Overview

Various aspects relate to a silicon (Si)-based photon sensor and excitedphoton spectrum analyzer, e.g., a differential Geiger-mode photonspectroscope. Various examples receive photons at a solid-statephotodetector that provides an output signal, then pass the outputsignal through a differentiator to provide a signal representative ofthe number or rate of photons striking the photodetector. Variousexamples count pulses or pulse widths of this representative signal todetermine the rate of photons striking the photodetector. Variousaspects can be used in flow cytometry, but are not limited thereto.Various aspects provide increased sensitivity and dynamic range comparedto prior schemes. Various aspects relate to Single Photon Spectroscopy,e.g., counting each individual photon striking a photodetector. Variousaspects can provide increased sensitivity for hyperspectral analysis,compared to prior schemes.

Laser induced fluorescence (LIF) analysis can be performed by photoncounting for basic materials in flow cytometry, microscopy, imaging, andmaterial science. Auto fluorescence (AFL) and photo bleaching phenomenain quartz, glass, sheath fluid, or other biological materials can bemeasured based on total photon counting over detectable spectral ranges.In order to investigate low level fluorescence, single photon spectralanalysis with high spectral resolution can be performed in the spectraldomain (i.e., across various wavelengths of light). Various aspectsherein include devices, systems, and methods for performing singlephoton spectrometry. Various aspects use a motorized monochromator and aphoton detector as described herein. Various aspects can measure thephenomena described in this paragraph, or optical characteristics ofglass, quartz, or other light-transfer or -blocking materials, e.g.,materials used in optical filters, lenses, or other components; inoptically-clear vessels or adhesives; or in optical stops.

Various aspects permit using single-photon detection in flow cytometry,microscopy, or imaging. Various examples permit using single-photondetection in physics and chemistry experiments, e.g., the evaluation ofmaterials such as optical components or other materials. Other examplesof materials can include liquids, e.g., reagents, fluorochromes,buffers, or consumable liquids such as oils, wine, beer, juice, orwater. Various examples of material evaluation permit measuringproperties of materials with higher sensitivity and speed than someprior schemes, and at multiple wavelengths of light. Some examplespermit evaluating properties of carbon nanotubes or other nanostructuredmaterials. Such properties can include perhaps absorbance,autofluorescence, or reflectance. Some examples include measuring suchproperties using techniques described herein, and comparing the measuredproperties to references (e.g., in a database) to determine qualitativecharacteristics of the materials. Such characteristics can include,e.g., whether the materials are genuine.

In a tested example, samples were illuminated by 405-nm laser light in a100 μm-diameter spot (after a bandpass filter) to reduce or eliminateaccelerated spontaneous emission (ASE). Excited fluorescence on theoptical axis was collected by a NA 0.125 lens and coupled to NA 0.22optical fiber (after a laser wavelength notch filter). In some examples,e.g., using a filter with an optical density of about six (OD6), anadditional notch or long pass filter can be used to separate laserillumination and induced fluorescence. The optical fiber was connectedto a motorized monochromator having a 500 nm/sec scanning speed over a200-900 nm spectral range. A synchronized stepping motor was used todrive a reflective grating. Photoelectrons were counted in a presetgating time for each spectral step selected. This permitted measuring atspectral steps of 1 nm and time intervals (gating times) of 0.1 sec.Counted photoelectrons were measured at specific photon energy levels.Such measurements can be performed in the spectral domain or the timedomain. In some examples, spectral steps can be between ˜0.1 nm and ˜10nm. In some examples, gating times can be between ˜1 ms and ˜10 s.

Various examples permit measuring or characterizing AFL spectra andbleaching phenomena, e.g., of basic materials used in flow cytometryinstruments. Some examples permit observing spectra such as Ramanspectra excited by laser wavelength, impurities, and molecularstructures in materials. Single photon spectroscopy as described hereincan provide increased sensitivity and wide dynamic range compared toprior schemes. Some examples can permit quantitative analysis offluorescence, which can permit investigation of material characteristicsas well as bio assays that exhibit lower levels of fluorescence than doassays testable by some prior techniques.

Various examples include a silicon (Si) photon sensor and single photonspectrometer that provide high sensitivity and permit quantitativeanalysis for fluorescence using relatively short measurement times.Single photon spectroscopy can permit next-generation cellular analysisand material improvement. Some examples can collect samples inmicrosecond-scale time windows via parallel spectral detection.

Various aspects relate to a wide-dynamic-range Si Photon DetectionSystem using a differential Geiger-mode sensor. Digital photon detectionbased on the Einstein-Planck equation can permit sensing small numbersof photons. However, for photodetection in the context of biology, e.g.,flow cytometry, some prior photodetectors do not provide a sufficientlywide dynamic range to effectively capture conjugated fluorescencesignals. Therefore, various flow-cytometry schemes use photocurrentdetection from a photomultiplier tube (PMT), solid-state photomultiplier(PM), or micro-PMT.

Various examples herein provide a Si photon sensor and electronics witha wide dynamic range, e.g., six orders of magnitude of dynamic range.Accordingly, sensors herein can be used in cytometry or otherbiological-analysis contexts.

Some examples use arrayed Si avalanche photodiodes (APDs) operating inthe Geiger mode. Incident photons on a pixel (e.g., a particular APD)produce electron-hole pairs. The corresponding current is magnified byseveral orders of electrons by breakdown of the electric field at p-njunctions. Charged electrons are quenched by a serial register in eachpixel, e.g., in ˜50 ns for a ˜100 kΩ quench resistor and a ˜500 fF pixelcapacitance. In prior schemes, the quenching time of a pixel defines theupper limit of photon pulse counting per second. The lower limit isdetermined by dark count or dark count deviation caused by thermaleffects. In general, the dynamic range of a multi-pixel Siphotomultiplier is dependent on the number of pixels.

Various examples herein collect a differential signal from a pixel. Thisstyle of collection is referred to herein as “differential Geiger mode.”Using differential signals permits detection of multiple photons from apixel even within the quenching time of that pixel. Shorter pulse pairresolution (less than 1 ns), as in various examples, can permitresolutions up to, e.g., one billion counts per second (1 Gcps). Variousexamples include a very-high-speed comparator and GHz-rate counterelectronics, which can permit detecting and counting sub-ns photonpulses. Various examples have a low dark count of 50 kcps/mm² at roomtemperature, in a tested configuration. Various examples provide furtherreduction of the dark count by actively cooling the sensor, e.g., usinga cooling unit. Various examples include a Peltier cooling unit and canachieve dark counts lower than e.g. 1 kcps, or about 100 cps at −30° C.,with 10 cps standard deviation. Various examples include system-controland measurement software.

Various example Si photon detection systems herein can evaluate fromsub-fW to nW range light power with theoretical linearity. Variousexamples provide a detectable sensitivity improved by about 3 orders ofmagnitude compared with prior photocurrent detection schemes. Variousexamples provide picosecond time resolution, which can permitmeasurement or analysis of individual molecular interactions in livecells. Some examples permit calibrating light power and analyzingspectrum as a measurement technology. Some examples include, e.g.,calibrating based on knowledge of the Si photon sensor photon-detectionefficiency (PDE) and reflective grating diffraction efficiency forparticular wavelengths. Various examples can permit measuring intrinsicauto fluorescence phenomena of optical and biologically-pertinentmaterials such as quartz, glass, pure water, or silica or polymer beads.

Various examples can measure individual photons, which may have very lowenergies. Values of photon energy versus wavelength for typical laserwavelengths in flow cytometry are: 405 nm: 740 THz 3.06 eV; 488 nm: 614THz 2.54 eV; 532 nm: 563 THz 2.33 eV; 594 nm: 504 THz 2.08 eV; 633 nm:473 THz 1.95 eV; 780 nm: 384 THz 1.58 eV. Owing to the small energy perphoton, the number of photons per pW can be expressed as megacounts persecond (Mcps), e.g.: 405 nm: 2.04 Mcps/pW; 488 nm: 2.46 Mcps/pW; 532 nm:2.68 Mcps/pW; 594 nm: 3.00 Mcps/pW; 633 nm: 3.20 Mcps/pW; 780 nm: 3.95Mcps/pW. In general, 1 pW is the lowest detection limit of a PMTphotocurrent signal. In general, a photon sensor has thermal noise,known as dark count, in the range of 1 cps to ˜1 Mcps. Dark count issensitive to temperature and determines the detection limit. Inaddition, dark-count standard deviation per second (r) and coefficientof variation CV (%) are considered as the resolution limit of lightintensity. Temperature control of the sensor can improve dark count andits standard deviation.

Photon counting is the digital measurement of light intensity withextremely high sensitivity and linearity. If a detected photon pulse isan ideal impulse with pulse width “zero” and dark count “zero,” thephoton detection system is ideal. Unfortunately, real photon pulses havefinite pulse width and waveform. The upper count rate is determined bypulse width and the lower limit by dark-count rate. Owing to pulseoverlapping, the true count value N and measured value M are describedas: N−M=NMt, where t is the pulse pair resolution. For example, with t=1ns, the error is relatively small up to 10 Mcps and a gradually largererror for higher count rates. If necessary, it is possible to correctthe measured value up to ˜1 Gcps by this model. 1 Gcps is equal to0.5/QE nW at 405 nm and 0.25/QE nW at 780 nm (QE=quantum efficiency).Therefore, it can be possible to achieve over six decades of magnitudeand linearity with dark count <1 kcps and pulse pair resolution 1 ns,e.g., at a maximum 1 Gcps measurement. However, photon pulse pairresolution in some prior schemes is longer than 10 ns. Furthermore,measured signals have fluctuations that produce counting thresholddeviation, causing additional error in some prior sensors.

Various examples can reach these levels of performance. Dark count rate(DCR) is another factor to determine sensitivity limit. In general, darkcount is proportional to sensor area. A smaller photocathode or sensoractive area usually reduces dark counts. Reducing sensor area by 90% mayreduce dark count by about 90%. However, prior schemes, e.g., of flowcytometer optics for fluorescence detection, have relatively largeaberrations as well as a large spot image due to broad wavelength andhigh NA collection lens without compensation. Various examples usereflective optics or optical fiber coupling to improve detection withsmaller sensors.

Sensor structure and material can also significantly contribute to darkcounts. Dark-count origins include thermal noise in the sensor orphotocathode. Materials with higher sensitivity in the IR region havehigher dark-count characteristics. For example, comparing bialkali andmulti-alkali photocathode materials for detection at extended longerwavelengths, multi-alkali shows a higher dark count. Various examplescontrol dark count and reduce signal deviations by temperature control.Peltier cooling can be used to reduce dark count. For cooling purposes,a smaller sensor is easier to implement.

Light intensity measurements with theoretical linearity can be performedby photon counting in the digital domain. Various examples permitanalyzing the photon spectrum or photon energy. Hyperspectral analysisis a useful technique for cellular analysis. Photon spectroscopy can beimplemented in at least two ways. One is in combination with a motorizedmonochromator and photon detector using a long capture time (>1 s). Arecent motorized monochromator has a wavelength scanning speed of 500nm/s. Because photon measurement for flow particles is in the μs to mstime domain, some examples include a parallel photon detection system.

Various examples permit biological fluorescence analysis using detectorshaving high sensitivity and wide dynamic range with linearity inwavelengths of interest, e.g., visible wavelengths or other wavelengths.Photon detection has sufficient sensitivity if dark count is low. Theupper dynamic range of photon detection is mainly determined by maximumcount capability per second. Various examples provide a shorter photonpulse width and reduced dead time, compared to some prior schemes.

Throughout this description, some aspects are described in terms thatwould ordinarily be implemented as software programs. Those skilled inthe art will readily recognize that the equivalent of such software canalso be constructed in hardware, firmware, or micro-code. The presentdescription is directed in particular to algorithms and systems formingpart of, or cooperating more directly with, systems and methodsdescribed herein. Aspects not specifically shown or described herein ofsuch algorithms and systems, and hardware or software for producing andotherwise processing signals or data involved therewith, can be selectedfrom systems, algorithms, components, and elements known in the art.

Steps of various methods described herein can be performed in any orderexcept when otherwise specified, or when data from an earlier step isused in a later step. Example method(s) described herein are not limitedto being carried out by components particularly identified indiscussions of those methods. Section headers are solely for ease ofreference and do not restrict the combinations covered by thisdisclosure. A feature described in a particular section can be used inconjunction or combination with features described in that section or inother sections.

Illustrative Configurations, Techniques, and Operations

FIG. 1 shows a block diagram of an example excited-photon spectrumanalyzer system, and an example graph (lower left) of output in countsper second (cps) as a function of wavelength in nanometers (nm) from400-800 nm. The example graph shows a visible range of 400 nm-800 nm,but this is not limiting. Various examples use a differential Geigermode for photon detection. Example detectors can include an array ofavalanche photodiodes having capacitive couplings (“C-couplings”) to asensor output (a “fast output”). In some examples, the fast outputs arewired in parallel, e.g., as shown in FIG. 36 . Example photon sensors,and related components, are described herein with reference to FIG.2A-15, 19, 21A-22, 27A-28, 32, 33 , or 35-37. For example, any of thefollowing SENSL SiPMs can be used: 10010, 10020, 10035, 10050, 30020,30035, 30050, or 60035. For example, an SiPM can have an active area of1 mm², 3 mm², or 6 mm²; a photodiode-cell size between 10 μm² and 50μm²; a fill factor between 25% and 75%; a photodiode count between 250and 3000; or a photodiode density between about 100 mm⁻² and about 1000mm⁻². “BP” is a bandpass filter; “notch” is a notch filter. “LaserBP” isa bandpass filter that will substantially pass the laser light; “Lasernotch” will substantially block the laser light. “Giga-counter” is anelectronic pulse counter, e.g., capable of counting at GHz rates.

The laser light L, or other electromagnetic radiation radiated into thesample, is referred to herein as “incident light.” Light or otherelectromagnetic radiation transmitted through the sample, or lightemitted from targets, fluorescent dyes (e.g., bound to targets), orother substances within the sample, is referred to herein as “resultantlight.” Resultant light can include electromagnetic radiation providedby, e.g., scattering, refraction, absorption, or rotation of the planeof polarization of the incident light, or any combination thereof.Throughout this document, the terms “scatter,” “scattering,” and thelike include refraction, absorption, and polarization rotation, asappropriate, unless otherwise explicitly noted. Resultant light caninclude forward-scattered (FS) light and side-scattered (SS) light. FSand SS have substantially the same wavelength as the light source, e.g.,the laser. Resultant light can also include fluorescent light, sincesuch light can be emitted by the sample or substances within the sample.Resultant light can be substantially directional (e.g., transmittedlight of the laser light L) or substantially omnidirectional (e.g.,fluorescence), or can have a selected or characteristic radiationpattern (e.g., a cardioidal variation of intensity as a function ofangle from the direction of the incident light).

Various example sensors or detection systems herein provide at least oneof: a fast output, an amplifier having a bandwidth >200 MHz, animpedance <200Ω, or a pulse width (pulse duration) of <5 ns. Forexample, bandwidth >200 MHz and impedance <200Ω can permit detecting 1ns-wide pulses. In some examples, sensors as described herein, e.g.,with reference to FIG. 2A-4A, 11A-11C, 32, 33 , or 35-37 can be used asthe illustrated “photon sensor.” Various examples of photon-countingspectral-analysis systems such as that shown in FIG. 1 can provide orinclude pulse clamping (e.g., FIG. 9, 10 , or 1517), a comparator (e.g.,FIGS. 7 and 9 ), or very-high-speed counting (e.g., FIG. 9 ).

Various example systems and techniques shown in FIG. 1 and elsewhereherein can permit collecting data and analyzing the data to determine,e.g., wavelength or time dependencies for phenomena such as photobleaching. Various examples permit detecting photon counts and, fromthose photon counts, together with at least one of photon energy,illumination power, optics NA, or sensor PDE, quantitatively estimatingand analyzing fluorescence characteristics. Various examples permitdetermining optical-filter performance, performing material AFLanalysis, performing color analysis, performing bio-assay analysis, orperforming cellular or particle analysis.

In the illustrated example, incident light from the laser strikes thesample. Resultant light is collected in the optical fiber and carried toa spectral-discrimination device (“spectral device” or “spectro unit”),e.g., a monochromator or polychromator. The illustrated example shows a“motor driven spectro unit,” e.g., a motorized monochromator. However,this is not limiting. In other examples such as some shown below inTable 1 and FIGS. 26B-29 , a motorized or non-motorized polychromatorcan be used, e.g., with an array sensor or a fiber array. Thespectral-discrimination device permits measuring different wavelengthsof light separately. In FIG. 1 , a single photon sensor successivelymeasures each wavelength of interest in the resultant light. In otherexamples, one or more photon sensor(s) or elements thereof canconcurrently measure respective wavelength(s) of the resultant light.The measurements can include rates of photon incidence on the photonsensor, e.g., in counts per second.

In various examples, a photon sensor can include a sensing element suchas a vacuum-tube based photomultiplier tube (PMT), a solid-state (e.g.,Si) photomultiplier tube (SSPMT, e.g., FIGS. 32 and 33 ), a siliconphotomultiplier (SiPM, e.g., FIGS. 2A-3B), an avalanche photodiode(APD), or a single-photon avalanche diode (SPAD). The term “tube” isused in reference to SSPMTs merely to distinguish them from SiPMs andother types of photon sensors. The term “tube” does require, that anSSPMT have any particular pressure (e.g., vacuum) or material containedin the SSPMT. In various examples, a photon sensor can include multiplesensing elements, e.g., an array of SSPMTs arranged on a printed circuitboard (PCB), or an array of SiPMs, APDs, or SPADs arranged on a wafer(e.g., FIGS. 2A-3B). In some examples, a photon sensor can includecombinations of any of these. SSPMTs, SiPMs, APDs, SPADs, and arrays ofany of those are examples of solid-state photon detectors (SSPDs). Theterm “SiPM” is used for brevity and can include other semiconductorphotomultipliers (SPMs) that include APDs or SPADs, e.g., APDs or SPADson InGaAs or SiC substrates, or other semiconductor types discussedbelow. In some examples, SiC is used for detection of ultravioletphotons.

Various examples can use different spectral-discrimination devices orphoton sensors. Nonlimiting example combinations can include thoselisted in Table 1. Throughout this document, “CH” refers to a channel ofoptical measurement. Examples of various components listed in Table 1are described herein with reference to FIGS. 1-6B, 9, 11A-11C, 17, 18,19, 20, 21A-21G, 22, 23, 24, 25 , or 26A-29.

TABLE 1 Number of Spectral Example light photon Type of photon Examplefeatures of Device extraction sensors sensor and mount some aspectsMonochromator Angular scan 1 Any SSPD listed wide λ range(200-900 nm);across slit, above (e.g., FIGS. Accurate FWHM; Flexible optical fiber 1or 26A) combination coupling, or sensing element Polychromator Linearscan 1 Any SSPD listed λ range (340~800 nm); across fiber above,receiving light simple mechanism; faster coupling via an optical fiberscan than angular; no 2^(nd) (e.g., FIG. 26B, harmonic linear scan)Polychromator Linear scan 1 Any listed above, As above; All in one;across sensing mounted to receive compact; lower cost element (e.g.,light directly from mounted polychromator (e.g., SSPD) FIG. 26B, array)Polychromator No movement; 32-64 SSPD array sensor, Parallel detection;High fixed fiber or group of discrete speed detection in μs array,32-64CH sensing elements. Mounted to receive light from respectiveoptical fibers (e.g., FIGS. 27A and 27B Polychromator No movement;128-256 SSPD array sensor, As above; Low dark count; fixed SiPM (ormore, mounted to receive All in one array e.g., 1024 or light directlyfrom 2048) polychromator (e.g., FIG. 26B, array)

FIG. 2A shows an example structure of a SiPM, which is an example of asolid-state photon detector (SSPD). An SiPM such as that shown canprovide small size, high quantum efficiency, lower bias voltage, lightdurability, insensitivity to magnetic fields, lower cost, and more,compared to a conventional PMT. In a PMT, an incident photon produces anelectron-hole pair in the photocathode, which is an electricalinsulator; vacuum and high voltage can permit capture of the inducedelectron. On the other hand, the electron-hole pair of a solid-statesensor such as that shown is produced in the p-n junction, which issemi-conductive material. The produced electron moves rapidly and theacceleration depends on the reverse bias electric field. The illustratedSiPM includes a photodiode (PD), e.g., an APD or SPAD, and a quenchresistor R, discussed below.

FIG. 2B shows various examples of principles of operation of exampleSiPMs. In the Geiger region of operation, the bias voltage issufficiently high in magnitude (e.g., >5×10⁵ V/cm) that, when a photonstrikes the sensor and releases a photoelectron, that electron canstrike other atoms and release additional electrons. Accordingly, asingle photon can trigger a cascade of electrons that causes the SiPM tobecome conductive and produce a detectable current. There are threephases of operation: P-I-N mode with gain=1, linear avalanche mode withgain˜100 and Geiger mode over break-down voltage with gain ˜10⁶. Geigermode is highly sensitive for incident photons owing to a high QE ˜0.8and gain >10⁶.

Once a photon has hit and triggered an avalanche, a quenching resistorin series with the PD is used to reset the PD without requiring activecircuitry. As the current increases, the quenching resistor's voltageincreases, and so the photodiode's voltage decreases. Once the currentincreases to the point that the photodiode's voltage is below the Geigerbreakdown threshold, conduction will substantially cease until thephotodiode's voltage has risen above the breakdown threshold and thenext photon impact event takes place. Quenching time, typically 50 ns,is called dead time because the detector will not fire even if struck bya photon. In order to expand the dynamic range, a Geiger mode sensor hasa structure arrayed as pixels (FIG. 3A). SiPM photon detectionefficiency PDE is defined as QE×ϵ_(geo)×ϵ_(trig), where ϵ_(geo) is thegeometrical fill-factor and ϵ_(trig) is the avalanche triggeringprobability. When a pixel is “fired,” a SiPM has secondary firephenomena called “cross-talk,” at adjacent pixels, and “afterpulse,” adelayed signal in the fired pixel. Various examples permit performingaccurate photon-count measurements even in the presence of thesefactors.

FIG. 2C shows an elevational cross-section of an example SiPM fabricatedon a wafer.

FIG. 2D shows an example of electric-field magnitude during operation ofan SiPM.

APD cells vary in dimension from 20 to 100 microns depending on the maskused, and can have a density of up to 1000/mm². Avalanche diodes canalso be made from other semiconductors besides silicon, depending on theproperties that are desirable. Silicon detects in the visible and nearinfrared range, with low multiplication noise (excess noise). Germanium(Ge) detects infrared to 1.7 μm wavelength, but has high multiplicationnoise. InGaAs (Indium Gallium Arsenide) detects to a maximum wavelengthof 1.6 ξm, and has less multiplication noise than Ge. InGaAs isgenerally used for the multiplication region of a heterostructure diode,is compatible with high-speed telecommunications using optical fibers,and can reach speeds of greater than Gbit/s. Gallium nitride operateswith UV light. HgCdTe (Mercury Cadmium Telluride) operates in theinfrared, to a maximum wavelength of about 14 μm, uses cooling to reducedark currents, and can achieve a low level of excess noise.

Silicon avalanche diodes can function with breakdown voltages of 100 to2000V, typically. APDs exhibit internal current gain effect of about100-1000 due to impact ionization, or avalanche effect, when a highreverse bias voltage is applied (approximately 100-200 V in silicon).Greater voltage can be applied to silicon APDs, which are more sensitivecompared to other semiconductor photodiodes, than to traditional APDsbefore achieving breakdown allowing for a larger operating gain,preferably over 1000, because silicon APDs provide for alternativedoping. Reverse voltage is proportional to gain, and APD gain alsovaries dependently on both reverse bias and temperature, which is whyreverse voltage should be controlled in order to preserve stable gain.SiPMs can achieve a gain of 10⁵ to 10⁶ by using Geiger mode APDs whichoperate with a reverse voltage that is greater than the breakdownvoltage, and by maintaining the dark count event rate at a sufficientlylow level.

Geiger-mode APDs produce relatively large charge pulse when struck by aphoton of the same amplitude no matter the energy of the photon. Whenreading out conventional APDs, noise of the preamplifier significantlydegrades timing and amplitude resolution performance for short (shorterthen ˜500 ns) light pulses. Compared to conventional APDs, SiPMs usingGeiger mode APDs provide much higher output amplitude, which can reducethe effect of preamplifier noise.

FIG. 3A is a graphical representation of a micrograph, and FIG. 3B is acircuit diagram, showing example configurations of SiPM arrays. Asshown, a sensor can include rows and columns of sensor elements. Asshown in FIG. 3B, each sensor element (or individual sensor elements)can include a quench resistor in series with a Geiger-mode avalanchephotodiode (APD), e.g., a SPAD. as in Example details of thesecomponents are shown in FIGS. 2A-2D. The sensor elements (“pixels” or“microcells”) can be electrically in parallel across a row, column, or2-D sensor array. As current flows through the APD in response to impactof a photon thereon, voltage across the quench resistor increases.Therefore, voltage across the APD decreases. When the voltage across theAPD drops below the Geiger threshold (FIG. 2B), the APD will cease toconduct and will be ready to detect another photon. This process isreferred to as “quenching.”

An array SiPM (e.g., using Geiger mode PDs) such as that illustrated canprovide high gain (e.g., >10⁶) and high sensitivity of photon detection.Such sensors can be relatively compact and can operate with relativelylow bias voltages, e.g., <25-70 V. Such sensors can be durable underlight exposure and can be relatively insensitive to magnetic fields.However, some prior schemes have a limited dynamic range due to thelimited number of pixels on the sensors and the dead time duringquenching, during which those prior sensors do not detect photons.Moreover, some prior sensors have relatively high dark count rates(i.e., appreciable output signal even when no light is incident) or canbe sensitive to temperature variations.

In some examples, the SiPM avalanche process is very fast, e.g., on theorder of picoseconds (e.g., 10 ps-100 ps). However, the quenching(“recharge” or “dead time”) process is comparatively slow, e.g., on theorder of nanoseconds (e.g., 50 ns-100 ns). As noted above, during someprior schemes, the sensor does not detect photons during the rechargeprocess. Accordingly, some prior sensors only provide a dynamic range ofabout three orders of magnitude.

Some examples sensors and detection systems herein permit using SiPMsfor biological photon detection applications that require wider dynamicrange, e.g., six orders of magnitude. Various examples detect theavalanche mode using a combination of a high pass filter and adifferentiation signal processing block. A tested example was able toperform multiple photon detection, even during the dead time.Accordingly, various examples can provide a dynamic range up to, e.g.,six orders of magnitude

FIG. 4A shows an example circuit using a differentiator (represented asan RC high-pass filter followed by a buffer (triangle)) to provide 1 nsphoton pulses. In some examples, the differentiator produces a timederivative (e.g., ΔV/s or ΔA/s) of the input signal (e.g., V or A).Using a differentiator can permit detecting multiple photons using asingle pixel, which can in turn increase dynamic range. The SiPM outputwaveform has a fast avalanche process measured in picoseconds and a slowrecharge process measured in nanoseconds. The high-pass filter andsignal differentiation permit detecting only the avalanche process.Experiments were performed using an ultrahigh-speed differentiatingcircuit with GHz-bandwidth amp or equivalent differentiation in pixelsto show multiple photon detection even during quenching dead time. Thismeasurement technique, and corresponding signal processing describedherein, are referred to for convenience and without limitation as“differential Geiger mode.” In some examples, a monolithic or on-chipcapacitor is used to perform filtering with reduced parasiticcapacitance and inductance compared to off-chip parts. In theillustrated example, the differentiator's input (on capacitor C) isconnected opposite the quench resistor from the APD. The illustrateddifferentiator can additionally or alternatively be connected to: theanode of the SSPMT shown in FIGS. 32 and 33 ; the Vbias or Ioutelectrodes shown in FIG. 35 ; or the Vbias, Fast out, or Iout/Commonelectrodes shown in FIG. 36 .

FIG. 4B shows an example output from the circuit of FIG. 4A, showingthat a single pulse produces a detectable spike in the differentiatedphoton signal output by the differentiator circuit.

FIG. 4C shows an example output from the circuit of FIG. 4A. As shown,the differentiated photon signal shows pulses even when a photon strikesthe detector during the dead time (quenching period). Although thedifferentiator is shown as attached on the anode side of the APDs, itcan additionally or alternatively be attached on the cathode side of theAPDs. Similarly, the quench resistor for an individual APD can beattached on either the anode or the cathode side of that APD.

An experiment was performed to measure a photon pulse using an SiPMwithout a fast out, e.g., similar to the configuration of FIG. 4A. Aunipolar pulse of FWHM ˜600 ps duration was detected, with a pulseheight of ˜50 mV.

An experiment was conducted to measure counts per second (cps) from asensor as a function of the power of the light incident on the sensor inpW. The experimental results showed a positive correlation between pWand cps. That correlation was substantially linear in certain regimes,e.g., 0-400 pW and 600-1400 pW. In some examples, accordingly, countsabove 250 Mcps can be provided and counted. An experiment was alsoconducted to measure signals output by a differentiator circuit, e.g.,as discussed herein with reference to FIGS. 4 and 5 . The experimentaldata showed discernable pulses.

FIG. 5 shows an example differentiator circuit including a high-passfilter section and a differentiation section. Various example ranges ofcomponent values are illustrated. The values of the componentsillustrated can be adjusted, e.g., to provide differentiation signalprocessing with a rise time T_(rise) corresponding to the timescale ofthe avalanche process with respect to a particular SiPM, e.g., 10 ps-100ps. In some examples, differentiation circuitry such as that shown isused with SiPMs or similar devices. Differentiation circuitry can beused to increase detection sensitivity, e.g., of photon detectors thathave asymmetric waveforms (e.g., ps-range rise times and ns-range fallor quenching times).

In some examples, differentiation signal processing can distinguishphotons within overlapped photon pulses. This can provide more accuratephoton counting, since photons that would otherwise be missed during anSiPM's dead time can be detected. In the illustrated circuit, an idealsquare pulse produces a Gaussian pulse after the high pass filter.Circuit parameters of the differentiation section can be adjusted toprovide separation of two partially-overlapping Gaussian pulses. Someexample circuit parameters can include:C _(HP) (Farad)=T _(RISE)/50  High-pass filter:C _(DIFF) (Farad)=(⅕)CHP=T _(RISE)/250  Differentiation filter:F0 (Hz)=1/(10 T _(RISE))  Primary corner frequency:avalanche process time; typical<100 ps  T_(RISE):

FIG. 6A shows an example input to the differentiation stage of thecircuit of FIG. 5 . FIG. 6B shows an example output from that circuit.As shown, the differentiation stage responds to two overlapping pulses(top), e.g., triggered by two separate photons, by providing a signalhaving two zero-crossings (bottom). Other thresholds than zero canadditionally or alternatively be used. As indicated by the double-endedarrows in the top plot, a multiple-photon pulse can have a wider pulsewidth (longer duration) than a single-photon pulse. Accordingly, a pulsewidth differential can be used to discriminate closely overlapped1-photon pulses from 2-photon pulses. Examples of pulse discriminationare described herein with reference to FIG. 14 .

FIG. 7 shows an example adaptive comparator circuit. The example circuitcan provide, e.g., adaptive reference (threshold) level changesfollowing pulse modulation. Due to AC-coupling of outputs from thedifferentiator section, large excursions of the signal baseline canoccur among rapid successive pulses. To reduce miscounts, theillustrated circuit causes threshold level(s) respond adaptively bytracking the signal baseline. An RF envelope detector, e.g., monolithicor made from discrete COTS components, produces a signal that closelyfollows the negative peak excursions of the input. An appropriate filterfurther smooths the signal. A constant offset is added to the envelopeto produce the tracking threshold level(s). The pulse is delayed, e.g.,through at least one of a long transmission line or a passive network,so that the pulse arrives to the comparator at the same time as themoving threshold(s).

FIG. 8 shows example inputs and outputs of the adaptive comparatorcircuit of FIG. 7 . Two thresholds are illustrated: a large-pulsethreshold and a small-pulse threshold. In some examples, the small-pulsethreshold is used to detect single-photon excitation (1PE) pulses andthe large-pulse threshold is used to detect double-photon excitation(2PE) pulses. A 2PE pulse can result from near-simultaneous absorptionof two photons, or from absorption of a single photon of a shorterwavelength.

FIG. 9 shows a block diagram of signal processing for photon detection,and related components. Various examples are designed to operate with atleast one of: an avalanche process ˜100 ps, a photon pulse width <1 ns,or a quenching time >50 ns. In some examples, output can be taken from,or referenced to, the cathode, the anode, or both. Although shown asconnected on the anode side of the APDs, the measurement electronics canadditionally or alternatively be connected on the cathode side.

FIG. 9 shows sequencing of signal processing. FIG. 5 showsdifferentiation before a comparator. FIG. 7 shows determination of thecomparator threshold. FIG. 9 shows an example overall flow of signalprocessing: photo current—HP—Pedestal clamping—differentiation (FIG. 5)—comparator (FIG. 7 ).

In some examples, the differentiation section can mix the signal withthe differential of the signal. This can accentuate overlapping peaks,increasing dynamic range.

In some examples, a pedestal correction factor between 0.1 and 0.15 canbe used. In some examples, the pedestal correction can be determinedbased on the light level (e.g., high vs. low) using feedback from theadaptive threshold determining unit.

In some examples, a full amplifier is used. In some examples, e.g., inwhich capacitance is implemented in each sensor pixel, a pre-amplifier(preamp) can be used without a full amplifier.

An experiment was conducted to measure single-pulse and multiple-pulsedetection. In the tested example, multiple photons were detected, evenduring the avalanche quenching time. Some prior schemes are unable todetect photons during the quenching time.

In some examples, the high pass filter permits detecting avalancheprocesses as <1 ns-width pulses, e.g., corresponding to a single photon.In some examples, the high pass filter output can be processed to detectmultiple photons even during a recharge process, unlike prior schemes.

FIGS. 10A and 10B show example circuit configurations of adaptivepedestal clamping circuits, e.g., as discussed herein with reference toFIG. 9 . In some examples, the illustrated circuitry can stabilize thepedestal level of short photon pulse signals. In some examples, as anumber of incident photons increases, a signal pedestal fluctuates dueto the high-pass filter. However, counting photon signals without thehigh-pass filter can lead to level deviations and a smaller signal,which can result in the limited dynamic range of some prior schemes.Therefore, some examples count the high-pass filter or differentiatoroutput and use pedestal clamping circuitry to reduce pedestal-levelvariation and to stabilize the inputs to the comparator.

Some examples improve the accuracy of the pedestal clamping by reducingthe phase difference between the pixel-signal output and thedifferentiator (“HF”) output. For example, components of the circuit canbe adjusted, or delay lines or other delay elements added, to keep thephase shift of the electronics below the pixel pulse width.

An experiment was conducted to measure data of pixel output signals,differentiator (“HF”) output signals, and signals after adaptivepedestal clamping. Tests were performed at a relatively lower count rate(lower optical power incident on the sensor) and at a relatively highercount rate. The tested adaptive-pedestal circuit provided a steadybaseline from which a threshold for determining counts could be set. Insome examples herein, once the baseline has been set by the adaptivepedestal clamping, the threshold determined by the adaptive thresholdunit can be applied to count pixels by counting times when thedifferentiator output crosses the threshold.

In some examples of a Gaussian profile having a full width at halfmaximum (FWHM) of 1.2 ns, pulse height and pulse resolution arenegatively correlated. For a photon pulse amplitude following a Gaussianform, comparator pulse resolution is correlated with count accuracy. Insome examples, to detect 1 ns pulse widths, 200 ps pulse resolution isused. Reducing the pulse resolution can reduce the measured count. Somecommercial universal counters have a pulse resolution of 4 ns, and some1 GHz digital oscilloscopes have a resolution of about 0.4 ns.Therefore, in some examples, a high-frequency counter, e.g., implementedusing an FPGA, is used to provide, e.g., 200 ps pulse resolution orbetter resolution.

FIG. 11A is a graphical representation of a micrograph of a portion ofan example SiPM.

FIGS. 11B and 11C show examples of estimation of the avalanche processtime, and internal structures of example SiPMs.

FIG. 12 shows simulated outputs of the circuit of FIG. 13 . The SPICEsimulation conditions were .tran 0.00001 0; mark=1000000000; space=0;PULSE(1.5 0.5 0.000002 0.00000001 0.00000001 0.000002 0.000004).

FIG. 13 shows an example filter section in an envelope-detector circuit(e.g., FIG. 7, 8, 10A-10B, 13 , or 14). The illustrated filter sectioncan be used, e.g., in the adaptive threshold circuit of FIG. 7 .

In some example envelope detectors, the diode is specified for GHzoperation with junction capacitance, e.g., about 1 pF-about 10 pF. Someexample envelope detectors include a transimpedance amplifier (TIA).Component values can be, in some examples: V2 1.4 V; L1 22 nH; R3 50Ω;C1 22 nF; R4 50Ω; R1 50Ω; C2 20 pF; R2 5 kΩ; C3 20 pF.

FIG. 14 shows an example pulse-width counter/discriminator circuit. Someexamples include various ways to distinguish pulse widths. In general, afixed comparator threshold level can be used to detect pulse widthdifference. As an example, the block diagram shown in FIG. 14 candiscriminate between single- and overlapped-photon pulses.

In some examples, illustrated circuits can detect two overlapping photonavalanches that result in a pulse that is longer (wider) than a“standard” pulse width of a single-photon pulse. For linearity of themeasurement system, some examples identify such pulses as being twophotons rather than one photon.

The illustrated examples use two comparators: Comp 1 and Comp 2. Thecomparators can be single-ended- or differential-output. Variouscomparator technologies can be used, e.g., technologies that allow forpropagation delays of hundreds of picoseconds.

FIG. 15 shows example timing diagrams of the circuit of FIG. 14 . In theillustrated examples, longer pulses are counted twice: once with thelower counter on Comp 1, and once with upper counter on Comp 2. Thetotal count is then the sum of the lower counter's count and the uppercounter's count. In illustrated examples, Comp 2 can only go high if thepulse is longer than one standard pulse-width, because Comp 2 is onlyenabled once a standard pulse-width's worth of time has passed after theonset of the pulse. This can prevent double-counting of single photonswhile still providing accurate counting of two overlapped photon pulses.

In some examples, Comp 1 is continuously enabled while measurements arebeing taken. Comp 2 is enabled by Comp 1, after a delay to. The inputsignal is also delayed by an amount less than to, so that it arrives atComp 2 one standard pulse-width before Comp 2 becomes enabled.Therefore, Comp 2's input signal is high when it becomes enabled only ifthe pulse is wider than one standard pulse-width. In some examples, thebuffer and delay line introduce a delay of td, the predeterminedstandard pulse-width). The propagation delay td is the time between whenthe input pulse arrives at Comp 1 and when comp 2 becomes enabled(t_(d)<t_(o)). The standard pulse-width varies by device and preamp. Insome examples, the standard pulse-width can be ˜400 ps-˜900 ps. Thestandard pulse-width can be selected empirically and embodiedphysically, e.g., in delay lines, or as a stored configuration value insoftware or firmware. For a long pulse, Comp 2 outputs a high level andstretches the output pulse in time, so that the counter can register thepulse.

The illustrated polarity is an example and is not limiting. For example,the counters can increment on an active-low or active-high clock.Suppose a positive rising pulse is applied to IN+ and a threshold isapplied to IN−. The comparator will go LO→HI as the positive pulsecrosses the threshold. This is appropriate for an active-high counter.For a negative pulse, the inputs would be flipped in order to continueusing an active high comparator. In some implementations there may bemultiple buffers or multiplexers for signal observation and sourcing;those muxes could invert the signal one or more times.

FIG. 16 shows an example spectrofluorometer configuration according tovarious prior schemes. Various prior schemes suffer from at least one ofthe following limitations. Use of an incoherent white light source(e.g., a xenon or halogen lamp) and two monochromators can result inlarge power consumption for the lamp, e.g., 150 W; low excitation powerthrough a slit; intensity that is difficult to measure; or difficultyobserving high-intensity phenomena like photo-bleaching. A perpendicularoptical layout between the sample and a fluorescence detector, used toavoid effects due to the excitation light, limit the configuration andshape of samples and sample cells that can be used. Use of an emissionmonochromator and photocurrent detection can prevent the use ofquantitative collecting optics, resulting in measured intensity valuesin arbitrary units rather than meaningful photometric units, and canprovide a low sensitivity due to photocurrent detection. Accordingly,some prior schemes do not provide quantitative fluorescence measurement,and can be bulky and expensive.

FIG. 17 shows a single-photon spectrometer configuration according tovarious examples herein. Illustrated is a coherent light source, e.g., alaser. Other lasers or sources that provide one or more specificwavelengths of light can be used. An illumination intensity control unitcan be used, and can include, e.g., at least one of a laser powercontrol unit, a polarizer, or a neutral density (ND) filter.

In some examples, illumination optics can include a lens (L); a lasernotch filter (NF), e.g., a dielectric filter, at a beam waist of thelaser beam or other incident light; and a laser bandpass filter (BP)between L and NF. A sample (SL) or other target to be measured can belocated adjacent to or otherwise proximal to the NF filter. This canprovide a double excitation effect by reflection of the laser beam offthe notch filter. Examples are discussed herein, e.g., with reference toFIG. 18 or 20 . Fluorescence detection optics downstream of SL and NFcan include additional notch filters, long-pass (LP) filters, lenses tofocus emitted resultant light, e.g., fluorescence, to an optical fiber,or any combination thereof. A motorized monochromator can select aparticular wavelength of light to direct to the photon sensor. Thephoton sensor and associated electronics, e.g., as in FIGS. 1-15 , candrive a counter to count pulses. The counter value can be read by, e.g.,a personal computer (PC) and software.

In some examples, meta-material absorption materials can be used toreduce effects due to stray light in a spectrometer. For example, acarbon-nanotube absorber can have <0.1% reflectivity. This can increasethe dynamic range of the measurement system.

Some examples use an optical bandpass filter (BPF). In various examples,a BPF can reduce or substantially eliminate accelerated (or amplified)spontaneous emission (ASE). In some examples, an incident laser beaminduces ASE photons with a broad spectrum, e.g., red-shifted slightlywith respect to the incident light. In an example, the ASE peak extendsfrom approximately 430 nm to approximately 465 nm. To improve theaccuracy of photon counting, ASE can be reduced by adding a bandpassfilter, e.g., centered at 405 nm (or other laser peak wavelength) with awidth of, e.g., 20 nm (e.g., 395 nm-415 nm). Data can be measured ofstray light and autofluorescence at various power levels, demonstratingthat the measurement system can effectively characterize even theperformance of some of its own optical components.

In a tested example, a SiPM was illuminated with a light source througha cover having a thickness of ˜3 μm. The measured data exhibited peaksat specific wavelengths, indicating that interferometric effects werepresent in the resultant light being measured. These effects can be dueto protective layers on the SiPM die or package. To reduce noise due tosuch interferometric effects, anti-reflective (AR) coating(s) can beused on the SiPM or optical elements associated therewith.

Additionally or alternatively, detectors as described herein can be usedto measure layer thicknesses using interferometric effects. An exampleoptical structure can include two interfaces (“1-2” and “2-3”) betweenthree layers (“1,” “2,” and “3”). In some examples, at least one of thelayers (e.g., layer 1, or layers 1 and 3) can be air. For example, Layer2 can represent a cover of the SiPM. The thickness d of Layer 2 can bedetermined based on the wavelengths (nm) of the peaks and valleys of theinterference pattern (λ_(n)), and on a refractive index n. In anexample, layer 1 is air (n≈1.0), layer 2 is a transparent layer havingrefractive index n, and layer 3 is a reflective layer, e.g., comprisingsilicon. Using measurement techniques herein can permit, e.g., measuringthe thickness d of the cover of the SiPM (layer 2) using diffractionpatterns. In various examples, detection circuitry as described herein(e.g., SiPM sensors and circuitry such as shown in, e.g., FIG. 9 ) canpermit measuring thickness of optical filters or other elements, e.g.,imaged onto a cover-less SiPM (or a SiPM with an AR-coated cover).

FIG. 18 shows illumination and collection optics according to variousexamples herein. “NF” is a notch filter. “FL” is fluorescence. Variousexamples permit readily aligning the laser beam and the filter/samplepositions. In some examples, the laser beam is focused on the NF filterto obtain a conjugate image, i.e., an image that follows an optical pathsymmetrical to the optical path of the incident illumination. In someexamples, the laser beam is reflected by the NF and excites the sampleagain. This provides a double excitation effect and roughly double thedetection sensitivity. For example, if the reflected laser beamilluminates a portion of the sample that has not yet been illuminated,another fluorescent photon may be emitted.

In some examples, the collection optics can have a consistent numericalaperture (NA). Therefore, the collection efficiency can be determinedquantitatively. Moreover, the optical fiber used to convey the collectedresultant light can couple to small sensor area effectively. In someexamples, the NA of the collection optics is smaller than the NA of thefiber. In some examples, a long pass filter is positioned between NF andfiber. In some examples, a filter can be used as discussed herein withreference to FIG. 24 . In some examples using a long pass filter toreduce laser leakage, only single-photon pulses are measured. In someexamples, an OD6 filter is not used.

Experiments were conducted to measure cps as a function of time forvarious tested examples. Tests were performed of Quartz/Glass photobleaching at a high level of exposure, as indicated by the total photoncount. The tests were conducted with a 405 nm laser emitting 500 μW anda 100 μm-diameter (φ) beam waist. Three tests were conducted,respectively with: a quartz 0.1 mm thick, cover glass 0.17 mm thick, andcover glass 0.1 mm thick. The plotted exposure data were collected overthe course of ten minutes (600 s). Quartz exhibited the lowest totalcount and intermediate photo bleaching, 0.1 mm cover glass exhibited thehighest total count and high photo bleaching, and 0.17 mm cover glassexhibited an intermediate total count and low photo bleaching(relatively little change in output over the course of the ten minutes).

FIG. 19 shows some components and optical paths of an example motorizedmonochromator. The illustrated monochromator can be configured toreceive broadband illumination via the entrance slit and provideillumination of a narrow bandwidth (e.g., only a few nm) via the exitslit. The motor can turn or otherwise adjust the grating or otheroptical components of the monochromator to select which wavelengths arecollected and counted.

There are various sources of stray light in a monochromator. Someinclude stray light from the entrance, stray light from the exit, andAFL from the inside walls of the monochromator. Other sources include2nd order and 0th order light from the grating, in configurations inwhich the 1st order light is normally used. To reduce stray light andimprove detection sensitivity when using monochromators and otheroptical enclosures, various examples use a metamaterial absorber. Forexample, carbon nanotubes (CNTs) with or arranged in metastructure canabsorb photons to a level of less than 0.1% reflectivity, compared to atypical anodized Al coating reflectivity of about 4%. Improvements inreflectivity can directly improve dynamic range. For example, areduction in reflectivity to 1% of its former levels can improve thedynamic range of a spectrometer by 100 times.

Measured data was collected at various wavelengths for a testedmonochromator. Calculated addresses for the wavelengths were comparedwith the measured data. The tested monochromator agreed with thecalculated results.

In some examples, systems such as that shown in FIG. 1 can providephoton counts for each of a plurality of wavelength intervals at each ofa plurality of times (or in each of a plurality of time windows). Forexample, the wavelength windows can be 0.1-10 nm, the scan times for aparticular wavelength can be 1-1000 s, and the exposures can be capturedover 1 ms-10.00 s. However, other values outside these ranges canadditionally or alternatively be used. In some examples, 1 s samplesover a long period of time, e.g., ≥600 s, can be used to measure photobleaching. An example is shown in FIGS. 24 26. In some examples, thedark count can be about 125 cps.

An experiment was conducted to measure data of a single-photon spectrumof a sheath fluid for a flow cytometer, as measured by a system such asthat shown in FIG. 1 including a motorized monochromator. The systembackground count was measured after installing, successively, a 440 nmlong-pass filter, a tube holder, and dry tubing. Measurements were thentaken after liquid sample filling to determine autofluorescence (AFL) ofa tested sheath liquid for a flow cytometer, e.g., as discussed hereinwith reference to FIG. 24 . The system was able to effectively measurethe sheath liquid, with cps peaking over 450,000 compared to abackground level after installing the dry tubing of less than 50,000.

FIG. 20 shows an example configuration of an optical system permittingwall-less fluorescence detection of liquid samples by coaxialillumination in a tube. In some examples of wall-less detection, novessel is used in a manner that would cause the vessel to exhibitautofluoresence. Abbreviations are as discussed herein with reference toFIGS. 17 and 18 .

Many materials exhibit autofluorescence (AFL), including high-gradequartzes and glasses. In some prior schemes, liquid samples require avial for measurement. However, measurements of the sample fluorescencecan be affected by AFL of the vial walls, i.e., photons emitted by thevial walls because of AFL. Therefore, wall-less capturing of sampleliquid permits fluorescence detection with reduced noise from vesselAFL. In some examples, the sample liquid is held by surface tensionwithin a 2-3 mm (inner diameter) glass tube, e.g., having relativelyflat inner surfaces and open ends. This size is nonlimiting; other sizescan be used as long as the sample is retained within the tube by surfacetension. Other materials than glass can be used for the tube, e.g.,quartz. The sample is illuminated along a coaxial direction, e.g.,substantially parallel to the walls of the tube. This can significantlyreduce the amount of incident light reaching the tube walls, so cansubstantially reduce measurement noise due to vial wall autofluorescence. In some examples, 10 μL-20 μL of sample fluid is used toprovide a 2 mm sample thickness. In some examples, e.g., as discussedherein with reference to FIGS. 30 and 31 , the tube can be a tube orwell in an array of tubes, e.g., a plate array. Plates can include,e.g., 96 tubes, 384 tubes, or more tubes. In some examples, the tube hasan open end and retains a fluidic sample (e.g., a liquid sample) bycapillary action.

In some examples, the tube, the laser source, or other components of ameasurement system are arranged so that the beam does not cross orintersect with the tube within a light-collection area associated withthe tube and the sample. In the illustrated example, thelight-collection area is a portion of the sample or its surroundingsfrom which resultant light is directed to the collection fiber by thecollection lens or other optics. If the beam crosses the tube away fromthe light-collection area, AFL photons will not be directed to thecollection fiber, so will not corrupt the measurement. In some examples,the light-collection area comprises at least the smallest cylindricalvolume that is coaxial with the incident light, that extendslongitudinally only far enough to encompasses the entirety of thesample, and that extends radially only as far as necessary to encompass(or, in some examples, reach) the outer walls of the tube.

In some examples, measurement systems as described herein can be usedfor automated drug screening or other high-volume biological measurementtasks. For example, in automated high throughput screening, multiplesamples can be arranged in separate tubes along the path of a laser. Thefluorescence of each sample can be measured individually, permittingmeasuring more samples in a given amount of time. The dynamic range andwavelength selectivity can permit performing advanced chemical analyses,such as drug screening, not supported by some prior schemes.

FIGS. 21A-21G show examples of aperture shape conversion fiber bundlesconfigured to provide increased spectral resolution and couplingefficiency. In some examples, a bundle of fibers is used to convey lightbetween the monochromator slits and other optical components.

FIG. 21A shows spectrum 2100, e.g., as spatially distributed by agrating such as that shown in FIG. 19 . Overlaid on spectrum 2100 areexample fiber-bundle shapes 2102 (circular) and 2104 (rectilinear, shortaxis along the spectrum).

FIG. 21B shows an example monochromator for selecting light of aparticular wavelength from a broadband source such as sunlight.

FIG. 21C is a cross-sectional view of an example of dense packing ofcircular optical fibers.

FIGS. 21D and 21E show cross-sections of fibers bundled into circularbundles 2102. Such configurations can be used, e.g., at the interfacesto the laser, lenses, sensors, or sample.

FIGS. 21F and 21G show cross-sections of fibers bundled intosubstantially rectilinear configurations 2104. Such configurations canbe used at the monochromator entrance and exit slits to improve spectralresolution.

In the illustrated example, bundles A and B (FIGS. 21D and 21E) arecircular bundles, so have aspect ratios (long side:short side) of 1:1.Bundles C and D (FIGS. 21F and 21G) are rectangular bundles. Bundles Cand D are examples of bundles having an elongated shape, e.g., bundlesfor which the best-fit ellipsoid is substantially non-circular. Forexample, for bundles C and D, the elongated shape is a substantiallyrectangular shape. For bundle D, the elongated (rectangular) shape hasan aspect ratio (long:short) exceeding 3:1, specifically, 700:186≈3.8.In some examples, the elongated shapes have aspect ratios of at least,or greater than, 2:1. Fiber bundles can provide improved powerefficiency while maintaining spectral selectivity compared to some priorschemes.

Some example monochromators have performance figures of merit expressedin spectral width vs. slit width, and exit intensity vs. slit width. Insome examples, narrower slit widths of the monochromator generallycorrespond with narrower wavelength bands into or out of themonochromator, which can increase spectral resolution. For example, awider slit has larger signal but less spectral resolution compared to anarrower slit. In some examples, a conversion fiber bundle converts fromcircular close packing to linear packing, e.g., as in FIG. 21 . This canprovide increased signal and increased spectral resolution.

FIGS. 21F and 21G show examples of proximal ends of bundles. A bundlecan receive, at its proximal end, at least one of a spatially dispersedplurality of wavelengths, e.g., from a monochromator exit slit or whilescanning across the output field of a polychromator (e.g., FIG. 26B).The distal end of the bundle can be arranged to provide light to an SPM,e.g., as in FIG. 27B.

An experiment was performed to measure data of a test of conjugateexcitation in Quartz 7980, and the effect of conjugated optics. Themeasurement equipment used for this test included a single core fiber.The sensor's dark count level was measured from 350-850 nm, as was thesystem's background count curve (higher than the dark count level acrossthe range, but less so at longer wavelengths). Results were measuredacross that range of one-pass excitation of the quartz material, andshowed a peak around 40,000 cps near 510 nm, compared to a backgroundlevel of about 19,000 cps. A spectral profile was then measured byconjugated excitation, in which the quartz material receives double theillumination due to reflection of the incident light off the notchfilter after passing through the quartz material the first time. The˜510 nm peak of the two-pass condition was ˜71,000 cps, over twice thecounts per second of the one-pass.

An experiment was performed to measure the spectral profile, aftersubtracting the background, of conjugate excitation in Quartz 7980. Anintensity profile was measured by conjugated excitation. A separateintensity profile was measured by one-pass excitation. Thedouble-excitation curve had a peak 2.57×higher than the peak of thesingle-excitation curve (51.7 kcps vs. 20.1 kcps).

An experiment was performed to measure data of cps by wavelength(350-510 nm) for, e.g., various diameters of fiber core. In the testedcondition, using a core diameter >0.6 mm, the spectral FWHM was largerthan the sampling resolution. The FWHM as a function of core diameterwas as shown in Table 2.

TABLE 2 Slit width FWHM (nm) 0.2 mm 1.5 0.3 mm 2.0 0.4 mm 2.8 0.5 mm 3.40.6 mm 4.0 0.7 mm 4.6 0.8 mm 5.0

FIG. 22 shows an end view of an example using a relatively large-corefiber and a slit aperture at the end of the fiber. This combination canprovide improved coupling efficiency and spectral resolution compared tovarious prior schemes. The slit aperture can provide increasedselectivity with reduced complexity compared to fiber bundles.

An experiment was performed to measure data of optical efficiency (inpercent) and of FWHM (nm) as a function of slit width in mm. As slitwidth increased, optical efficiency and FWHM both increased. Below aslit width of 0.6 mm, efficiency dropped more rapidly as slit widthdecreased than was the case above 0.6 mm. The experiment showed thatFWHM was controlled by the input core size.

FIG. 23 shows components of an example optical system supporting on-axisand perpendicular measurement, in any combination. The illustratedcomponents can additionally or alternatively be applied at other anglesthan those shown, e.g., 45°. In some examples, the illustrated samplecan be located in a flow chamber, e.g., as discussed herein withreference to FIG. 24 . In some examples, detectors using a monochromatoras described herein (e.g., FIG. 17, 19, 21B, or 26A), or a polychromatoras described herein with reference to FIG. 26B, can be used to measurethe on-axis and perpendicular resultant light. Some examples can use aslit and large core combination (FIG. 22 ) with perpendicular detection.The on-axis and perpendicular configurations are shown for purposes ofexample; single photon spectroscopy as described herein can be appliedat any angle with respect to the incident beam. For example, on-axisdetection can be used with transparent samples and conjugate excitation.Perpendicular detection can be performed without a filter, so can havereduced spectral loss.

The Influence of excited AFL depends on optics configuration. In-linedetection to laser incident direction may be more affected thanperpendicular fluorescence detection. In the case of confocalfluorescence imaging or flow cytometry with longer gate period for smallparticles, optics AFL can be reduced for higher contrast and accuratedetection.

An experiment was performed to measure data of photon spectra of twodifferent quartz samples, in cps over the wavelength range (in nm). Inthe tested example, the two types of quartz were distinguishable fromeach other using single-photon measurements.

The experimental data were collected in a step mode. In some examples,counting can be performed in, e.g., a step mode or a scan mode. In stepmode, the monochromator can be adjusted to a particular wavelength(e.g., a 1 nm window), photons can be collected for a predetermined time(e.g., 1 s) while the monochromator is substantially fixed with respectto wavelength, and the adjusting and collecting operations can berepeated to collect respective photon counts for each of a plurality ofwavelengths.

In scan mode, by contrast, the monochromator can be adjusted to changethe wavelength continuously at a predetermined rate. For example, themonochromator can scan a 2 nm-wide window across the spectrum, and datacan be collected, e.g., substantially continuously, or at points orintervals in time associated with predetermined wavelengths (e.g., thewindow centered on 400 nm, 405 nm, 410 nm, . . . ).

FIG. 24 is a perspective view schematically showing an exampleflow-cytometry system 3800 that can be used with sensors and detectionsystems described herein, e.g., FIG. 23 . System 3800 includes a flowcell 3802. The flow cell 3802 includes a flow chamber 3804, and is, atleast in part, transparent or substantially transparent to irradiationsuch as light L and resultant light such as light L_T-FS and L_T-SS. Forclarity, only part of flow chamber 3804 is shown. Further details ofvarious configurations of flow chamber 3804 and other parts of a flowsystem 330 (FIG. 3 ) are discussed below with reference to FIGS. 3A, 3B,and 5-10B. As shown, the flow cell 3802 can be 2 mm thick along thedirection of propagation of light L.

A sheath flow SH flows into the flow cell 3802 from an inlet port IN1.For example, saline, which is an isotonic liquid, or water, can be usedas the sheath flow SH. However, the sheath flow SH is not limited tosaline, but various types of liquid such as water, other aqueoussolutions (whether isotonic or not), and organic solvents can be used.In various examples, the sheath flow SH also flows into the flow cell3802 from at least one additional inlet port, shown as IN3.

Further, a sample flow SM including microparticulate samples or othertargets to be analyzed flows into the flow cell 3802 from an inlet portIN2. For example, saline, which is an isotonic liquid, can be used asthe sample flow SM. However, the sample flow SM is not limited tosaline, but various types of liquid such as water, other aqueoussolutions (whether isotonic or not), and organic solvents can be used.The inflow pressure of the sample flow SM can be higher than or lowerthan the inflow pressure of the sheath flow SH. Flow chamber 3804 orother fluid channels in flow cell 3802 can be arranged so that thecenter of the sample flow is the fastest and the flow velocityapproaches zero at the walls of the flow channel 3804. This can causetargets to be hydrodynamically focused, i.e., positioned by the fluidflow, substantially in the center of the sample flow. In the illustratedexample, the fluid flows SM and SH are provided by a fluidic supply3806.

The inlet ports IN1, IN2, IN3 can be bored, molded, or otherwise formedin the flow cell 3802. In an example, the flow cell 3802 includes glassor quartz. For example, flow channels (e.g., flow chamber 3804) can beformed by micro-blasting of quartz sheets. Ports IN1, IN2, IN3 can bedrilled out of the quartz sheets. Other etching and boring techniquescan be used to form flow channels, inlets, and other features. Forexample, sample channels, including flow chamber 3804, can be etched,and sheath channels can be micro-blasted using a mask to define thedesired pattern. In other examples, channels and other cavitiesdescribed herein can be injection molded, molded using other techniques,bored, or etched.

The sheath flow SH and the sample flow SM merge in the flow chamber3804, so that a flow FL is provided in which the sample flow SM issubstantially hydrodynamically focused with the sheath flow SH, e.g.,around the sample flow SM, or arranged in other hydrodynamic-focusingconfigurations. The flow FL can be discharged to the outside of the flowcell 3802 in some examples.

An optical source 3808, e.g., a laser or other illumination source, canprovide light L aimed, focused, or otherwise directed to irradiate thetargets entrained within the sample flow SM. Laser light L can be atleast partly transmitted or at least partly scattered, providingresultant forward-scattered light L_T-FS and resultant side-scatteredlight included in L_F-SS. Targets, e.g., chromophores bound to targetmolecules of interest, can fluoresce, producing resultant fluorescentlight also included in L_F-SS. A detector 3810, e.g., an on-axisdetector (FIG. 23 ), can detect light L_T-FS. A detector 3812, e.g., aperpendicular detector (FIG. 23 ), can detect light L_F-SS. Variousexamples can use one detector or more than one detector. Detectors canbe placed at any angle with respect to the axis of the light L.

In some examples, optical source 3808 can include at least one componentdescribed in at least one of FIG. 1, 16, 17, 18, 19, 20, 21, 22, 23 , or25. In some examples, at least one of detector 3810 or detector 3812 caninclude at least one component described in at least one of FIG. 1-5, 7,9-11C, 13, 14, 17-23, 25, 28 , or 32-37. For example, optical source3808 can include a laser and conjugate focusing optics, and detector3810 can include a motorized monochromator, a SiPM using Geiger-modePDs, and a differential detection circuit (FIGS. 4A-6B).

In some examples, a controller (“CTL”) 3814 can control operation of thefluidic supply 3806 or the optical source 3808. In some examples,controller 3814 can receive information, e.g., photon counts, fromdetectors 3810, 3812.

Example Features

Various examples relate to multiple photon detection, e.g., with <1 nspulse width, e.g., by avalanche signal processing. Various examplesrelate to spectrum analysis. Various examples relate to or include atleast one of the following features, in any combination:

1. High pass filter Signal Processing: E.g., C_(HP) (Farad)=T_(RISE)/50,e.g., t=100 ps, c=2 pF. E.g., CHP (Farad)=T_(RISE)/25˜100

2. Adaptive pedestal clamping by ADD signal processing—Add two signalsusing a gain that stabilizes the pedestal level. In some examples, thetwo signal phase difference is smaller than 10 pulse pairs inresolution.

3. Differential Signal Processing of Photon Pulse: e.g.,C_(DIFF)(Farad)=(⅕)CHP=T_(RISE)/250. In some examples, the primarycorner frequency F0 (Hz)=1/(10 T_(RISE))

#3, using Differential Amplitude: C_(DIFF)(Farad)=(⅕)C_(HP)=T_(RISE)/250; Primary corner frequency F0 (Hz)=1/(10 T_(RISE))

#3, using Differential Pulse Width: Two comparators at a threshold levelof photon pulses; pulse width discrimination by pulse width comparison.

4. Adaptive comparator: Adaptive Envelope reference level determined asconstant+envelope signal.

5. Comparator pulse resolution: smallest resolvable pulse smaller thanpulse width at 70% level of Pulse height.

6. Photon Spectrum Analyzer including at least one of the following:Excitation Laser; focus lens; bandpass filter (laser wavelength); sampleillumination; collection lens; notch filter; fiber-motorizedmonochromator; fiber-photon detection.

#6, including a scanning monochromator for synchronized spectrum windowΔλ at x seconds.

#6, including photon counting per each Δλ, and repeated exposures.

#6, configured to provide data indicating n×Δλ vs. photon count as ahistogram.

#6, configured to provide data indicating a counting trend at a specificwavelength (photon energy).

7. Spectrometer including a coherent-light illumination unit, amonochromator, and a single photon detector.

#7, where the optics includes illumination optics and collection opticsthat convey the light to an optical fiber.

#7, where the optics are arranged in an in-line layout betweenillumination and detection.

#7, where the illumination optics focus the laser beam on a reflectivelaser notch filter. This way, the light excites the sample twice: oncebefore reflection and once after. The sample is located close to thelaser beam waist and the notch filter.

#7 or the preceding, where the Illumination optic include a laser bandpass filter to reduce laser spontaneous emission (e.g., ASE).

#7, where the collection optics collects fluorescence photon in specificsolid angle defined by a NA.

#7, where the monochromator IN and OUT fiber bundles have slit shapeswith close packing at the monochromator.

The preceding, where the fiber bundles have circular shapes away fromthe monochromator.

8. Tube Trapping of a liquid sample. This can reduce vial AFL.

9. Fluorescence photons scanned by a motorized monochromator forspectral analysis.

10. Data is analyzed for wavelength and time dependence, e.g.,photo-bleaching.

11. Estimating and analyzing fluorescence characteristics quantitativelybased at least in part on photon count, photon energy, illuminationpower, optics NA, or sensor PDE.

12. Any of the foregoing example features, combined or used withcomponents described herein with reference to FIG. 2A-2D, 4A-4C, or32-34.

13. Examples described herein with reference to FIG. 2A-2D, 4A-4C, or32-34.

Various aspects described herein provide detection and quantitativemeasurement (e.g., counts) of photons, e.g., using an SiPM or SSPMT. Atechnical effect is to measure emissions of fluorescent targets, e.g.,in a sample fluid. A further technical effect is to effectively detectphotons that strike a SiPM during the quench/recharge/dead time of thatSiPM.

In some examples, SiPM or SSPMT Photon Detectors as described herein canprovide improved sensitivity and linearity of fluorescence analysis. Adynamic range of six orders of magnitude can be measured, compared tothree orders of magnitude for conventional photocurrent analysis. Insome examples, flow cytometry systems herein can apply high power laserbeam illumination to a sample in order to analyze materialcharacteristics at high intensity illumination. Some examples can detectphoto-bleaching phenomena using single photon analysis, unlike someprior schemes. Some examples can reduce auto fluorescence compared tosome prior cytometers, e.g., by removing AFL effects due to a quartzflow chamber, sheath fluid, water, or calibration microbeads. Someexamples permit cellular analysis by determining material AFL andproviding very-low-fluorescence detection, e.g., of microparticles orsingle molecules.

Example Data-Processing System

FIG. 25 is a high-level diagram 3900 showing the components of anexample data-processing system 3901 for analyzing data and performingother analyses described herein, and related components. The system 3901includes a processor 3986, a peripheral system 3920, a user interfacesystem 3930, and a data storage system 3940. The peripheral system 3920,the user interface system 3930, and the data storage system 3940 arecommunicatively connected to the processor 3986. Processor 3986 can becommunicatively connected to network 3950 (shown in phantom), e.g., theInternet or a leased line, as discussed below. Devices above (e.g., thePC in FIG. 1 , the system in FIG. 9 , the PC in FIG. 17 , controller3814 shown in FIG. 24 , the PC shown in FIG. 34 , or other processingsystems herein) can each be or include one or more of systems 3901,3986, 3920, 3930, or 3940, and can each connect to one or morenetwork(s) 3950. Processor 3986, and other processing devices describedherein, can each include one or more microprocessors, microcontrollers,field-programmable gate arrays (FPGAs), application-specific integratedcircuits (ASICs), programmable logic devices (PLDs), programmable logicarrays (PLAs), programmable array logic devices (PALs), or digitalsignal processors (DSPs).

Processor 3986 can implement processes of various aspects describedherein. Processor 3986 and related components can, e.g., carry outprocesses for detecting photons, collecting count data from counters,operating a laser (e.g., adjusting the laser power), operating amonochromator (e.g., to scan across a wavelength band), or operating afluid supply or other components of a flow-cytometry system as in FIG.24 .

Processor 3986 can be or include one or more device(s) for automaticallyoperating on data, e.g., a central processing unit (CPU),microcontroller (MCU), desktop computer, laptop computer, mainframecomputer, personal digital assistant, digital camera, cellular phone,smartphone, or any other device for processing data, managing data, orhandling data, whether implemented with electrical, magnetic, optical,biological components, or otherwise.

The phrase “communicatively connected” includes any type of connection,wired or wireless, for communicating data between devices or processors.These devices or processors can be located in physical proximity or not.For example, subsystems such as peripheral system 3920, user interfacesystem 3930, and data storage system 3940 are shown separately from theprocessor 3986 but can be stored completely or partially within theprocessor 3986.

The peripheral system 3920 can include or be communicatively connectedwith one or more devices configured or otherwise adapted to providedigital content records to the processor 3986 or to take action inresponse to processor 186. For example, the peripheral system 3920 caninclude digital still cameras, digital video cameras, cellular phones,or other data processors. The processor 3986, upon receipt of digitalcontent records from a device in the peripheral system 3920, can storesuch digital content records in the data storage system 3940. In theillustrated example, the peripheral system 3920 permits the processor3986 to control fluidic supply 3806 and optical source 3808. Theperipheral system 3920 also permits the processor 3986 to receive datafrom detector(s) 3902, e.g., detectors 3810 or 3812. In some examples,the peripheral system 3920 also permits the processor 3986 to control aspectral discriminator 3960, e.g., a motor or other drive that operatesa monochromator to select the particular wavelength band output by themonochromator (e.g., FIG. 19 or 26 ).

The user interface system 3930 can convey information in eitherdirection, or in both directions, between a user 3938 and the processor3986 or other components of system 3901. The user interface system 3930can include a mouse, a keyboard, another computer (connected, e.g., viaa network or a null-modem cable), or any device or combination ofdevices from which data is input to the processor 3986. The userinterface system 3930 also can include a display device, aprocessor-accessible memory, or any device or combination of devices towhich data is output by the processor 3986. The user interface system3930 and the data storage system 3940 can share a processor-accessiblememory.

In various aspects, processor 3986 includes or is connected tocommunication interface 3915 that is coupled via network link 3916(shown in phantom) to network 3950. For example, communication interface3915 can include an integrated services digital network (ISDN) terminaladapter or a modem to communicate data via a telephone line; a networkinterface to communicate data via a local-area network (LAN), e.g., anEthernet LAN, or wide-area network (WAN); or a radio to communicate datavia a wireless link, e.g., WIFI or GSM. Communication interface 3915sends and receives electrical, electromagnetic, or optical signals thatcarry digital or analog data streams representing various types ofinformation across network link 3916 to network 3950. Network link 3916can be connected to network 3950 via a switch, gateway, hub, router, orother networking device.

In various aspects, system 3901 can communicate, e.g., via network 3950,with a data processing system 3904, which can include the same types ofcomponents as system 3901 but is not required to be identical thereto.Systems 3901, 3904 can be communicatively connected via the network3950. Each system 3901, 3904 can execute computer program instructionsto operate measurement systems or capture measurements as describedherein, or to communicate measurement data, e.g., via network 3950.

Processor 3986 can send messages and receive data, including programcode, through network 3950, network link 3916, and communicationinterface 3915. For example, a server can store requested code for anapplication program (e.g., a JAVA applet) on a tangible non-volatilecomputer-readable storage medium to which it is connected. The servercan retrieve the code from the medium and transmit it through network3950 to communication interface 3915. The received code can be executedby processor 3986 as it is received, or stored in data storage system3940 for later execution.

Data storage system 3940 can include or be communicatively connectedwith one or more processor-accessible memories configured or otherwiseadapted to store information. The memories can be, e.g., within achassis or as parts of a distributed system. The phrase“processor-accessible memory” is intended to include any data storagedevice to or from which processor 3986 can transfer data (usingappropriate components of peripheral system 3920), whether volatile ornonvolatile; removable or fixed; electronic, magnetic, optical,chemical, mechanical, or otherwise. Example processor-accessiblememories include but are not limited to: registers, floppy disks, harddisks, tapes, bar codes, Compact Discs, DVDs, read-only memories (ROM),erasable programmable read-only memories (EPROM, EEPROM, or Flash), andrandom-access memories (RAMs). One of the processor-accessible memoriesin the data storage system 3940 can be a tangible non-transitorycomputer-readable storage medium, i.e., a non-transitory device orarticle of manufacture that participates in storing instructions thatcan be provided to processor 3986 for execution.

In an example, data storage system 3940 includes code memory 3941, e.g.,a RAM, and disk 3943, e.g., a tangible computer-readable rotationalstorage device or medium such as a hard drive or solid-state drive(SSD). Computer program instructions are read into code memory 3941 fromdisk 3943. Processor 3986 then executes one or more sequences of thecomputer program instructions loaded into code memory 3941, as a resultperforming process steps described herein. In this way, processor 3986carries out a computer implemented process. For example, steps ofmethods described herein, blocks of the flowchart illustrations or blockdiagrams herein, and combinations of those, can be implemented bycomputer program instructions. Code memory 3941 can also store data, orcan store only code.

Furthermore, various aspects herein may be embodied as computer programproducts including computer readable program code (“program code”)stored on a computer readable medium, e.g., a tangible non-transitorycomputer storage medium or a communication medium. A computer storagemedium can include tangible storage units such as volatile memory,nonvolatile memory, or other persistent or auxiliary computer storagemedia, removable and non-removable computer storage media implemented inany method or technology for storage of information such ascomputer-readable instructions, data structures, program modules, orother data. A computer storage medium can be manufactured as isconventional for such articles, e.g., by pressing a CD-ROM orelectronically writing data into a Flash memory. In contrast to computerstorage media, communication media may embody computer-readableinstructions, data structures, program modules, or other data in amodulated data signal, such as a carrier wave or other transmissionmechanism. As defined herein, computer storage media do not includecommunication media. That is, computer storage media do not includecommunications media consisting solely of a modulated data signal, acarrier wave, or a propagated signal, per se.

The program code includes computer program instructions that can beloaded into processor 3986 (and possibly also other processors), andthat, when loaded into processor 3986, cause functions, acts, oroperational steps of various aspects herein to be performed by processor3986 (or other processor). Computer program code for carrying outoperations for various aspects described herein may be written in anycombination of one or more programming language(s), and can be loadedfrom disk 3943 into code memory 3941 for execution. The program code mayexecute, e.g., entirely on processor 3986, partly on processor 3986 andpartly on a remote computer connected to network 3950, or entirely onthe remote computer.

Further Example Embodiments, Features, and Experimental Results

An experiment was performed to measure PE characteristics, in cps by mV,at bias voltages from 27 V-29 V, by 0.5 V. As bias voltage increased, agiven mV level produced higher CPS. The five tested curves showedgenerally the same shape (a double waterfall), but shifted to the right(+mV) and slightly up (+cps) as bias voltage increased.

An experiment was performed to measure photo bleaching, indicated as thecps of the spectral peak as a function of accumulated exposure in mJ.The observed photo-bleaching occurs over a period of time, as theexposure accumulates. In the tested example, cps peaked at 23 kcps at<10 mJ of exposure, and declined to 2.6 kcps after about 280 mJ ofexposure.

An experiment was performed to determine the amount or extent of photobleaching using normal and reverse scans. In the illustrated example,measurements were collected from short wavelengths to long wavelengths(normal scan), 420 nm-670 nm. Subsequently, measurements were collectedfrom long wavelengths to short (reverse scan). The difference betweenthe two measurements at a given wavelength indicates the photo-bleachingat that wavelength. In some examples, the reverse scan can be performedbefore the normal scan. In the tested example, the normal scan peaked at(446 nm, 110 kcps). The corresponding peak on the reverse scan was ataround 55 kcps. The curve shapes were generally the same.

An experiment was performed to measure spectra of a multi-ion dischargelamp in cps as a function of wavelength from 400-700 nm using techniquesdescribed herein. The measurements showed strong peaks at ˜430 nm and˜540 nm. Techniques described herein can additionally or alternativelybe used to measure other types of lamps, e.g., to test conformance ofstandard lamps or other sources to corresponding illuminants.

The experimental data covered more than three orders of magnitude (frombelow 10³ cps to above 10⁶ cps). Accordingly, some prior schemes havingat most three orders of magnitude of dynamic range would not be able toeffectively measure the lamp to provide the experimental data. Moreover,some prior lamp-characterization systems provide results in arbitraryunits. As discussed above, counts measured by some example detectorsdescribed herein can be readily converted to physical units. This canmake the collected data applicable to a wider range of uses than dataprovided by some prior schemes.

An experiment was performed to measure spectra using techniquesdescribed herein, in cps as a function of wavelength. A tungsten-halogenlight source was measured without a filter, and found to have broadbandemission over the range 400-700 nm. Two filters were measured: “525 BPA”and “525 BPB”. The wide dynamic range (cps as high as ˜1.6×10⁷) andpossibility to report results in physical units permit characterizingfilters more accurately than in some prior schemes. This can permitimproving the quality of data from flow cytometry, since filtercharacteristics can significantly affect cytometric results.

An experiment was performed to measure spectra (log scale) of variouscolors of physical test targets: Semi-transparent sky blue; Largelyopaque violet; Semi-transparent red; Semi-transparent pink;Semi-transparent orange; Semi-transparent yellow; Semi-transparentgreen; Semi-transparent blue.

The physical test targets were plastic POST-IT flags of differentcolors. The measurements were able to span more than four orders ofmagnitude, from a system baseline of ˜10⁴ cps to a peak measurement˜1.2×10⁸ cps. This is much more dynamic range than some prior schemes.

FIG. 26A shows a perspective of an example motorized monochromator.Example monochromators are described herein with reference to FIGS. 17,19, and 21 .

FIG. 26B shows an elevational cross-section of an example polychromator.The polychromator can, e.g., disperse incoming light across to afiber-optic cable bundle, e.g., as in FIG. 21 or 22 . Additionally oralternatively, the polychromator can include or be connected to a motionsystem (e.g., a linear stage) that translates a fiber or fiber bundleacross the spectrum spatially distributed by the grating.

FIG. 27A shows an example 42-channel (“42 CH”) linear fiber arrayconnector 2700. The illustrated connector includes a bundle 2702 of 42optical fibers arranged along a line.

FIG. 27B shows output fibers fed by a polychromator such as that shownin FIG. 26 . Connector 2700 can connect to the polychromator and carrythe output fibers in bundle 2702. As noted by the color labels,different output fibers are carrying different wavelengths of outputlight.

FIG. 28 shows an example SiPM linear array configured for high-speed(e.g., μs) single-photon spectrum detection. The array can be used as aphoton sensor or optical detector in various configurations herein,e.g., FIG. 1, 9, 17, 18, 20, 24 , or 25. As shown, the array includesmultiple blocks of SiPM pixels (e.g., 250 pixels each, as 5 px×50 px).Each SiPM pixel can include a PD and a resistor, e.g., as discussedherein with reference to FIG. 2A, 3A, 3B, 4A, or 11A-11C. Each block ofpixels extends substantially along an axis that is substantially notparallel with (e.g., is substantially perpendicular to, or is at least40° from) a direction (horizontal) across which the spectrum is spread.Multiple blocks are arranged along the spectrum-spread direction (e.g.,256 blocks for a 256-channel array). Any number of blocks, or of pixelsper block, can be used. In the illustrated example, the array is orincludes a monolithic SiPM array. In some examples, electronics, e.g.,as in FIG. 4A, 5, 7, 9, 10A-10B, 13 , or 14, can be integrated within adie or package holding the SiPM array. In some examples, multiple arrayor electronics dice can be integrated within a package. The illustratedarray has 256 blocks (shown as strips extending vertically), eachcorresponding to one output channel. The blocks are packed togetherhorizontally. Each illustrated block can have a width of 100 μm, for atotal array width of ˜25.6 mm. Each block can be, e.g., 1 mm long. Eachblock can include 250 pixels, e.g., arranged in five rows of 50 pixelseach (as shown in the inset). In some examples, the blocks can beseparate blocks, e.g., spaced apart or abutted. In some examples, theblocks can be portions of an area sensor having a regular 2-D array ofSiPM pixels. In some examples, the linear array can comprise an areasensor.

In some examples, the dark count of a SiPM sensor is proportional tosensor area. Therefore, using small blocks (e.g., 1.0×0.1 mm as shown)can reduce the dark current (e.g., to one-tenth that of a 1×1 mmsensor). Cooling, filtering, or other techniques herein can furtherreduce the dark count.

Dark-count rate (DCR) is the lower limit of sensitivity and largelydetermines the dynamic range of photon counting. A cause of high darkcounts is thermal noise in p-n junctions. Thermal electrons areamplified in a similar manner to incident photons. Unlike photo currentnoise (thermal noise, shot noise, amp noise, and so on), the dark countcan easily be subtracted from the evaluated count rate. A Peltier cooleror other cooling unit can be arranged proximal to the SiPMs or otherSSPDs and configured to cool the SSPDs to reduce dark count. Otherexamples of coolers include forced-air ventilation coolers, e.g.,cooling heatsinks applied to the SiPMs; liquid-cooling systems; orcooling systems using dry ice or other compounds with freezingtemperatures below 0° C., either in direct contact with the SSPDs to becooled or arranged so that the vapor from those compounds is broughtinto thermal contact with the SSPDs to be cooled.

A 1 mm² SiPM sensor was tested. At temperatures higher than 25° C., theDCR was over 100 kcps, but was reduced to 2 kcps at −10° C. Variousexamples provide a DCR <1 kcps by cooling the sensor below −20° C.Experimental evidence showed that a DCR of 100 cps was attained bydry-ice cooling at −50° C. A counting range from 1 kcps to 1 Gcpsprovides a six-orders-of-magnitude dynamic range with theoreticallinearity in the digital environment. Temperature control also providesimproved data accuracy, stability, and reproducibility. The measuredvalue of DCR at 4° C. was 5 kcps, the standard deviation per second, r,was 10 to 50 cps, and the coefficient of variation was 0.2-1.0%,respectively.

FIG. 29 shows an isometric view of an example optical system and array.Resultant light is shown for clarity of explanation as a collimatedbeam, although this is not required. The illustrated optics collectresultant light and spread the light over the array (five blocks areshown, but any number can be used). The optics causes differentwavelength bands of light to strike each block (“H spreading”). For aparticular block, the optics can spread the light within thecorresponding wavelength band across the block to increase sensitivityor dynamic range of the detection system (“V spreading”). In someexamples, H and V spreading can be performed by separate optics, e.g., abeam expander for V spreading followed by a grating for H spreading. Insome examples, only H spreading is performed (e.g., for beams alreadysubstantially as wide as the blocks are long). V spreading canadditionally or alternatively be performed by directing the beam ontothe sensor at an angle other than normal to the sensor, e.g., from belowor above in the illustrated configuration, to expand the width of thebeam as projected on the sensor.

Accordingly, the optics can be configured to spread out differentwavelengths of the resultant light along the same direction the blocksare spread out (horizontal, as illustrated) and to spread out lightwithin a particular wavelength along a second direction substantiallydifferent from that direction (vertical, as illustrated), e.g., adirection perpendicular to or more than 40° from that direction. Theillustrated optics can include at least one slit, grating, beam shaper,beam expander, prism, mirror, or lens, in any combination. In anexample, optics includes a spectral discriminator, e.g., a grating orprism, to perform the H spreading, followed by a slit or a lensarrangement to perform the V spreading. The illustrated orientation, andthe terms “H” and “V,” are not limiting; other orientations or spatialarrangements can be used.

In some examples, the optics can include a toroidal mirror grating forthe H spreading. Additionally or alternatively, the optics can include aplurality of dichroic mirrors configured to reflect respectivewavelengths to respective blocks or groups of blocks. Additionally oralternatively, the optics can include prism(s).

FIG. 30 shows an example system for high-throughput sample measurementor analysis. Insets are indicated using dash-dot lines. A plate array,e.g., a substantially transparent microwell plate, can hold a pluralityof samples, e.g., fluidic samples. In some examples, e.g., as discussedherein with reference to FIG. 20 , the plate can include open tubes, andsamples can be held in the tubes by capillary action. A laser beam orother incident light can be directed into each tube, e.g., successivelyin a raster scan across the plate. Additionally or alternatively,multiple wells can be illuminated at once with respective laser beams orother incident light, e.g., from multiple light sources or from onesource split, e.g., using partially-silvered mirrors, diffractiongratings, or other optical elements, into multiple beams.

Resultant light from the sample, e.g., fluorescent light or other lightas in FIG. 1, 17, 18, 20, 23 , or 24, can travel out of the indicatedtube (e.g., microwell), e.g., substantially in the direction of travelof the laser or other incident light. This can reduce crosstalk betweenwells. Other directions of resultant-light travel can additionally oralternatively be used. A detector, e.g., as in FIG. 1, 17, 18, 20, 21,23, 24, 25, 28 , or 29, can detect the resultant light. One tube at atime can be measured, or multiple tubes can be measured concurrentlyusing multiple detectors.

In the illustrated example, the system has R rows of wells and C columnsof wells. In some examples, the system is configured with a detector foreach row, or for each column, and an irradiation system configured toprovide incident light to each tube in a column, or each tube in a row,respectively. This can permit measuring an entire row or an entirecolumn at once, which can increase throughput.

FIG. 31 is an isometric view of tubes in a portion of a plate array suchas that in FIG. 30 . Illustrated are six tubes and, for each, arespective path of the laser beam or other incident light.

Still Further Example Embodiments and Features

FIG. 32 shows a plan view of an example solid-state siliconmicroelectromechanical system (MEMS) photomultiplier tube (PMT), e.g.,sold by HAMAMATSU. The illustrated SSPMT is an example of an SSPD, andcan be used instead of or in addition to SiPMs in measurement systemsdescribed herein. Unlike a conventional PMT with metal dynode structure,the micro-PMT dynode is made by a Si MEMS process, which accuratelyproduces a small and thin structure. When a photon pulse from amicro-PMT is amplified by a high-speed preamp, it is possible to obtaina photon pulse of 4-5 ns. Example pulse waveforms from such micro-PMTsshow lower noise and distortion than some prior PMTs from smaller inputcapacitance owing to short distance and small area. In addition, a smallphotocathode area (e.g., 1×3 mm) can achieve very low dark-count ratesat room temperature. According to the velocity distribution of cascadeelectrons, the photon pulse for the PMT can show a continuousphotoelectron (PE) level. The combination of low dark count and narrowerpulse width can provide a photon detection system with a wider dynamicrange. Some PMTs provide gain and dynamic range control on the outputsfor both photocurrent and photon mode, permitting calibrating thedetected value to the absolute power level for each measurementcondition. In some examples, the output from the SSPMT is a current, andan external transimpedance amplifier (TIA) is used to provide avoltage-based photon signal. In some examples, the SSPMT includes or ispackaged with a TIA (e.g., in a system-in-package or system-on-chipconfiguration) and so can produce a voltage-based photon signal. Invarious examples such as those shown in FIG. 1, 17, 18, 20, 21A-24 , or26A-34, in order to collect fluorescence light and couple to the sensor,fiber- or aberration-free optics can be used.

In some examples, an SSPMT does not exhibit a significant quench orrecharge time, unlike an SiPM. For example, there is no structure insome SSPMTs that is required to recharge after a pulse. Therefore, someexamples that use an SSPMT and not an SiPM do not use differentiationcircuitry such as that shown in FIGS. 4A and 5 .

FIG. 33 shows an elevational cross-section of the PMT of FIG. 32 . Asshown, the PMT has glass covers on the top and the bottom. In someexamples, incoming light may reflect off both the top and the bottom ofa glass layer due to the difference in indices of refraction between theoutside air, the glass, and the inside environment of the PMT (e.g.,vacuum or neutral gas). Similarly, light may reflect off both sides of aprotective layer (e.g., ˜3 μm thick) over an Si base used in a SiPM(e.g., FIG. 2C). This can produce interferometric effects that affectSSPD output. Therefore, in some examples, the protective layer, glass,or other layer(s) through which light travels have anti-reflective (AR)coatings or surface structures, e.g., with one or more layers. In someexamples, at least one surface of at least one layer in a SiPM or SSPMThas a metamaterial coating or structure, e.g., a surface nanostructure,configured to reduce reflection at that surface.

FIG. 34 shows an example photon-counting measurement system, e.g., usingSSPMTs such as shown in FIGS. 32 and 33 . The illustrated components andparameters are illustrative and not limiting. In the case of 10 kcpsphotons at 405 nm, the intensity is approximately 5 fW. This is roughly1000 times higher sensitivity than is obtained with a conventionalphotocurrent approach. Various examples of photon counting candistinguish each photocurrent noise cause, permitting more accuratecellular analysis, e.g., of basic material in flow cytometry andbiology. Some examples include optical or other components such asdescribed herein with reference to FIG. 1 . Similar to FIG. 1 , incidentlight from the laser reaches the sample, and resultant light from thesample is focused by the collection lens into the optical fiber. Theoptical fiber carries the resultant light (e.g., broadband) to a spectrounit (motor-driven, as shown, or other examples such as those in FIG. 1). A photon sensor, e.g., an SSPMT or other SSPD, measures the resultantlight at various wavelengths. The illustrated example shows anamplifier, e.g., a TIA, that amplifies the photon signal. Other examplespectral discriminators and photon sensors are described herein withreference to Table 1.

FIG. 34 shows autofluorescence (AFL) evaluation optics in a testedexample. An excitation wavelength of 405 nm is the shortest wavelengthin the visible region with excitation energy that provides a fullvisible spectrum longer than the laser wavelength. A laser spot 100 μmin diameter illuminates the sample behind a bandpass filter to removestimulated spontaneous emission in the laser beam. Excited fluorescencephotons are collected by an NA-0.125 lens and coupled to an NA-0.22optical fiber through a 405-nm notch filter and a long-pass filter toremove excitation photons. Detection occurs at 420-900 nm and count isgiven as total number of photoelectrons (PEs) withoutdetection-efficiency correction.

Assuming that excited fluorescence is emitting uniformly to any solidangle, the total number of emitted photons is estimated as 1000 timesthe measured PE number because of lens collection efficiency ( 1/250)and sensor PDE (¼).

Excited autofluorescence intensity is roughly proportional toilluminating power and sample thickness under fixed optics. In order tocompare autofluorescence, an excitation coefficient k is defined as thedetected PE number per μW illumination for a 1-mm sample thickness(PEcps/μW mm). Interestingly, quartz, glass, and many materials show AFLand photobleaching. An experiment was performed to check the excitationcoefficient before and after photobleaching. Photobleaching is difficultto observe with the noncoherent light source in a conventionalfluorescence spectrometer. Lasers can provide very high illuminationintensity, over 10⁶ J/m², which is not proportional to totalillumination energy (J). Measurement is first dark count, system AFL,and dry vial for liquid, and finally the sample to calculate a countonly for that sample.

As an example, an excitation coefficient of 1000 cps/pW mm means thatthe total number of emitted photon is estimated as 1 Mcps (1 k×1 kcps)under measurement conditions. A photon measurement of 1 Mcps representsabout 1 pW of incident flux captured by the sensor. Illumination at 405nm/1 μW contains 2.04 Gigaphotons. This equals 1 Mcps/2 Gcps, ˜ 1/2000;it takes 2000 illuminating photons to produce a single emitted photon.Using a measured excitation coefficient, it is possible to estimate theAFL from the illumination level. A material with k=1000 cps/μW mm emits1 k×1 kcps=1 Mcps, ˜1 pW AFL under 1 mW illumination. Severalmaterial-evaluation results are shown in Table 3. Table 3 shows themeasured excitation coefficient per 1 μW/405 nm exposure and 1-mm samplethickness for basic materials in flow cytometry. Measurements were takenwith conditions 405 nm, 1 μW, 100 μm spot, and NA 0.125.

TABLE 3 Excitation coefficient Material k (cps/uW mm) Remark Flow cellquartz 3500-1500 Photobleach before/after Highest grade 700-300Photobleach quartz before/after Distilled water 250 Tube trapping Sheath1120 Tube trapping Clean polymer bead 2210 Tube trapping 1 pm in waterClean silica bead 10,700 Tube trapping 1 pm in water Y-G highlighter1,180,000 Tube trapping Slide glass 4700 1 mm thick borosilicate Coverglass 3200 (530) 0.17 mm thick borosilicate

Many materials for flow cytometry exhibit autofluorescence, e.g.,distilled water, sheath, or clean beads. Yellow-Green dye diluted inwater for flow check in a cytometer has a count of 1.2 Mcps/pW mm,meaning roughly one emitted photon for every two excitation photons inthe illuminated volume. In order to reduce the influence of AFL from thetube, a trapping method involving illumination along the coaxialdirection for liquid-sample evaluation can be used, as discussed hereinwith reference to FIG. 20 .

FIG. 35 shows a silicon photomultiplier in which the anodes of an arrayof photodiodes are connected to a common ground electrode and thecathodes of the array are connected via current limiting resistors to acommon bias electrode for applying a bias voltage across the diodes.

FIG. 36 shows an example SiPM in which a third electrode is capacitivelycoupled to each photodiode cathode to provide a fast readout of theavalanche signals from the photodiodes. When the photodiode emits acurrent pulse, part of the resulting change in voltage at the cathodewill be coupled via mutual capacitance into the third electrode. Using athird electrode for readout avoids the compromised transient performanceresulting from the relatively large RC time constant associated with thebiasing circuit.

In the illustrated example, the quench resistors and the fast-readoutcapacitors are connected to the respective cathodes of the photodiodes.In other examples, the quench resistors and the fast-readout capacitorsare connected to the respective anodes of the photodiodes. Theillustrated example shows a 1-D arrangement of photodiodes. However, thesame circuit can be used in a 2-D array such as that of FIG. 3A or 11Aby, e.g., wiring all the cathodes in each row together, and to all thecathodes in the other rows (and likewise for the anodes and the fastoutput electrodes). In some examples, a 2-D array can be segmented,e.g., into multiple groups of rows, the photodiode circuitry in eachgroup connecting to a single cathode and a single anode, and the groupshaving respective, different cathodes and respective, different anodes.

In some examples, the readout electrode is insulated by Silicon Oxide oranother dielectric from all the elements of the SiPM structure but hasweak capacitive coupling to each photocell. Such capacitive couplingprovides fast, partial (1-10% or so) injection of the signal charge(which is of the order of 0.15 pC/photocell) when the photocell isfiring. The capacitance of the third electrode toward other electrodesof the SiPM stays rather low, of the order of 10% of total SiPMcapacitance. To implement this electrode, a transparent conductor (forexample very thin gold, or a transparent conducting oxide such as IndiumTin Oxide) or a grid-like structure comprising a non-transparentconductor may be used. The third electrode may be galvanically isolatedfrom the first and second electrode. The signal in the third electrodemay be induced electrostatically. The third electrode may be coupled toa cathode of the photodiode, for example via a capacitive coupling. Thesignal in the third electrode may be induced by capacitive couplingbetween the third electrode and the photodiode cathodes. The capacitanceof the third electrode with the photodiode cathodes may be between 2%and 20% of the total silicon photomultiplier capacitance. Thecapacitance of the third electrode with the photodiode cathodes may beapproximately 10%.

FIG. 37 shows an equivalent circuit showing a manner of operation ofvarious aspects such as that shown in FIG. 36 . The current source I1represents the avalanche current of the diode, C1 the capacitance of thephotodiode and C5 the capacitance between the third electrode and thephotodiode. The remaining network represents a biasing and readoutcircuit. In the illustrated example, the current through R7 simulatesthe readout, unlike some prior schemes using R3. The current through R7can respond more quickly in both rise and fall than the current throughR3. Various examples include reading the output signal from the thirdelectrode using a wideband amplifier with low input impedance. Thewideband amplifier may have a bandwidth greater than 200 MHz and mayhave an input impedance of less than 200 ohms. Various examples includereading the output signal from the third electrode using a currentintegrating charge amplifier to provide an output proportional to thenumber of photodiodes firing within a period of time of interest.

Example Clauses

Various examples include one or more of, including any combination ofany number of, the following example features. Throughout these clauses,parenthetical remarks are for example and explanation, and are notlimiting. Parenthetical remarks given in this Example Clauses sectionwith respect to specific language apply to corresponding languagethroughout this section, unless otherwise indicated.

A: Signal-processing apparatus comprising: a differentiator (e.g., FIG.1, 4A-6B, or 9) configured to provide a differentiated photon signalbased at least in part on a photon signal associated with a photonsensor; and a crossing detector (e.g., FIG. 1, 4A-6B, 7, 9, 10A-10B, or14) configured to provide a count signal representing a number of timesthe differentiated photon signal crosses a threshold level.

B: The apparatus according to paragraph A, further comprising ahigh-pass filter (e.g., FIG. 5 ) configured to receive an output signalof the photon sensor and provide the photon signal.

C: The apparatus according to paragraph A or B, further comprising: ahigh-pass filter (e.g., FIG. 5 ) configured to receive an output signalof the photon sensor and provide a filtered signal; and apedestal-clamping circuit (e.g., FIG. 5, 9, 10A-10B, 14 , or 15)configured to determine the photon signal by adjusting a level of thefiltered signal based at least in part on an envelope of the filteredsignal.

D: The apparatus according to any of paragraphs A-C, wherein thecrossing detector further comprises an adaptive envelope reference unit(e.g., FIG. 5 or 7-9 ) configured to provide the threshold level byoffsetting an envelope level of the differentiated photon signal by apredetermined offset level.

E: The apparatus according to any of paragraphs A-D, further comprisinga silicon photomultiplier (SiPM) (e.g., FIG. 1-4C, 11A-11E, 25, 32, 33 ,or 35-37) configured to receive resultant light from a sample and, inresponse, provide the photon signal.

F: The apparatus according to any of paragraphs A-E, wherein: theapparatus further comprising a silicon-photomultiplier array sensor(FIG. 28 ) configured to receive resultant light from a sample and, inresponse, provide the photon signal; the array sensor comprises multipleblocks associated with respective sensor channels and arranged along afirst direction; and each block (e.g., a single row of pixels, or atwo-dimensional array of pixels) comprises a plurality of siliconphotomultiplier pixels (e.g., SiPM sensor pixels as in FIG. 2A-4D,11A-11E, 32, 33 , or 35-37) (e.g., each block can be a freestandingblock, or a portion of an area sensor).

G: The apparatus according to paragraph F, further comprising optics(FIG. 29 ) configured to: spread out different wavelengths of theresultant light along the first direction (e.g., a spectraldiscriminator); and spread out light within a particular wavelengthalong a second direction substantially different from the firstdirection (e.g., a beam shaper).

H: The apparatus according to any of paragraphs A-G, further comprisinga solid-state photomultiplier (e.g., a micro-photomultiplier tube,μPMT™, by HAMAMATSU, or other SSPMT) configured to receive resultantlight from a sample and, in response, provide the photon signal.

I: Signal-processing apparatus comprising: a pulse detector configuredto provide a pulse-width signal based at least in part on a photonsignal associated with a photon sensor; and a pulse counter configuredto provide a count signal based at least in part on both a number ofpulses of the pulse-width signal and respective widths of individualpulses of the pulse-width signal (e.g., FIG. 14 or 15 ).

J: The apparatus according to paragraph I, further comprising: a firstcounting unit configured to count the pulses of the pulse-width signal;a second counting unit configured to count those pulses of thepulse-width signal that have a pulse time greater than a predeterminedtime (e.g., FIG. 14 or 15 ).

K: The apparatus according to paragraph J, wherein the second countingunit comprises: a comparator (“COMP 2”) configured to provide a signalindicating that a pulse of the pulse-width signal has crossed apredetermined threshold; a delay circuit (“DELAY t0”) configured toenable the comparator substantially the predetermined time after a startof the pulse of the pulse-width signal; and a counter (“Upper Counter”)configured to count outputs of the comparator.

L: A measurement system, comprising: an optical source configured toirradiate a sample; a solid-state photon detector (SSPD) (e.g., asilicon photomultiplier, SiPM, or solid-state photomultiplier tube,SSPMT) configured to receive resultant light from the sample and, inresponse, provide a photon signal; and a photon counter configured toreceive the photon signal and, in response, provide a count signal,wherein the photon counter comprises: a differentiator configured toprovide a differentiated photon signal based at least in part on thephoton signal; and a crossing detector configured to provide the countsignal representing a number of times the differentiated photon signalcrosses a predetermined threshold level (e.g., FIG. 1, 17, 18, 20,21A-21G, 23-25, 28 , or 29) (in some examples, the SSPD can include ananti-reflective coating) (in some examples, optical components of themeasurement system can be coated with an optical absorption material,e.g., carbon nanotubes).

M: The system according to paragraph L, wherein: the optical sourcecomprises a laser or other source configured to emit substantiallymonochromatic light towards the sample; and the system comprises amonochromator (e.g., FIG. 19 or 26A-26B) configured to successivelyprovide a plurality of wavelength bands of the resultant light to theSSPD.

N: The system according to paragraph M, further comprising a drive(e.g., a motor; FIG. 1, 17, 19, 21A-21G, or 25) configured to cause themonochromator to successively provide the plurality of wavelength bands.

O: The system according to any of paragraphs L-N, comprising:illumination optics (e.g., FIG. 1, 17, 18 , or 20) configured tosubstantially focus the light onto the sample.

P: The system according to paragraph O, wherein: the optical sourcecomprises a laser configured to provide a laser beam having a primarywavelength; the illumination optics comprise a laser bandpass filterupstream of the sample along a direction of travel of the laser beam,wherein the laser bandpass filter is configured to absorb light atwavelengths substantially different from the primary wavelength; theillumination optics comprise a laser notch filter configured tosubstantially block light at the primary wavelength, wherein the lasernotch filter is arranged downstream of the sample along the direction oftravel of the laser beam; the illumination optics are configured tofocus the laser beam on the laser notch filter; and the illuminationoptics are configured to focus a beam waist of the laser beam proximalto the sample and the notch filter (e.g., FIG. 18 or 20 ).

Q: The system according to any of paragraphs L-P, further comprisingcollection optics comprising an optical fiber and configured to conveythe resultant light to the SSPD via the optical fiber.

R: The system according to paragraph Q, further comprising illuminationoptics, wherein the illumination optics, the sample, the collectionoptics, and the SSPD are arranged in an in-line layout.

S: The system according to paragraph Q or R, wherein: the systemcomprises a monochromator configured to successively provide a pluralityof wavelengths of the resultant light to the SSPD; and the collectionoptics comprise: a bundle of optical fibers; and a retaining structureconfigured to arrange the bundle of optical fibers substantially in anelongated shape at an exit of the monochromator and substantially in acircular shape at a port facing the SSPD.

T: The system according to paragraph S, wherein the elongated shape is asubstantially rectangular shape having an aspect ratio (long:short)exceeding 3:1.

U: The system according to any of paragraphs L-T, wherein: the opticalsource comprises a laser or other source configured to emit a beam oflight towards the sample; and the system comprises a tube configured toretain the sample and arranged so that the beam does not cross orintersect with the tube within a light-collection area associated withthe tube and the sample (e.g., FIG. 20 ) (in some examples, the systemcan be configured so that the beam does not cross or intersect with thetube).

V: The system according to any of paragraphs L-U, wherein the photoncounter comprises signal-processing apparatus as recited in any ofparagraphs A-K.

W: A method, comprising detecting resultant light from a sample usingdetection equipment recited in any of paragraphs A-V.

X: The method according to paragraph W, further comprising irradiatingthe sample using irradiation equipment recited in any of paragraphs A-V.

Y: A method, comprising: receiving a photon signal representingresultant light from a sample; providing a differentiated photon signalbased at least in part on the photon signal; determining a number oftimes the differentiated photon signal crosses a predetermined thresholdlevel; and providing a count signal representing the number of times.

CONCLUSION

This disclosure is inclusive of combinations of the aspects describedherein. References to “a particular aspect” (or “embodiment” or“version”) and the like refer to features that are present in at leastone aspect. Separate references to “an aspect” (or “embodiment”) or“particular aspects” or the like do not necessarily refer to the sameaspect or aspects; however, such aspects are not mutually exclusive,unless so indicated or as are readily apparent to one of skill in theart. The use of singular or plural in referring to “method” or “methods”and the like is not limiting.

Although some features and examples herein have been described inlanguage specific to structural features or methodological steps, it isto be understood that the subject matter herein is not necessarilylimited to the specific features or steps described. For example, theoperations of example processes herein are illustrated in individualblocks and logical flows thereof, and are summarized with reference tothose blocks. The order in which the operations are described is notintended to be construed as a limitation unless otherwise indicated, andany number of the described operations can be executed in any order,combined in any order, subdivided into multiple sub-operations, orexecuted in parallel to implement the described processes. For example,in alternative implementations included within the scope of the examplesdescribed herein, elements or functions can be deleted, or executed outof order from that shown or discussed, including substantiallysynchronously or in reverse order.

Each illustrated block can represent one or more operations that can beimplemented in hardware, software, or a combination thereof. In thecontext of software, the operations described herein representcomputer-executable instructions stored on at least onecomputer-readable medium that, when executed by one or more processors,enable the one or more processors to perform the recited operations.Accordingly, the methods and processes described above can be embodiedin, and fully automated via, software code modules executed by one ormore computers or processors. Generally, computer-executableinstructions include routines, programs, objects, modules, codesegments, components, data structures, and the like that performparticular functions or implement particular abstract data types. Someor all of the methods can additionally or alternatively be embodied inspecialized computer hardware. For example, various aspects herein maytake the form of an entirely hardware aspect, an entirely softwareaspect (including firmware, resident software, micro-code, etc.), or anaspect combining software and hardware aspects. These aspects can allgenerally be referred to herein as a “service,” “circuit,” “circuitry,”“module,” or “system.” The described processes can be performed byresources associated with one or more data processing systems 3901, 3904or processors 3986, such as one or more internal or external CPUs orGPUs, or one or more pieces of hardware logic such as FPGAs, DSPs, orother types of accelerators.

Conditional language such as, among others, “can,” “could,” “might” or“may,” unless specifically stated otherwise, are understood within thecontext to present that certain examples include, while other examplesdo not include, certain features, elements or steps. Thus, suchconditional language is not generally intended to imply that certainfeatures, elements or steps are in any way required for one or moreexamples or that one or more examples necessarily include logic fordeciding, with or without user input or prompting, whether certainfeatures, elements or steps are included or are to be performed in anyparticular example.

The word “or” and the phrase “and/or” are used herein in an inclusivesense unless specifically stated otherwise. Accordingly, conjunctivelanguage such as, but not limited to, at least one of the phrases “X, Y,or Z,” “at least X, Y, or Z,” “at least one of X, Y or Z,” and/or any ofthose phrases with “and/or” substituted for “or,” unless specificallystated otherwise, is to be understood as signifying that an item, term,etc., can be either X, Y, or Z, or a combination of any elements thereof(e.g., a combination of XY, XZ, YZ, and/or XYZ). As used herein,language such as “one or more Xs” shall be considered synonymous with“at least one X” unless otherwise expressly specified. Any recitation of“one or more Xs” signifies that the described steps, operations,structures, or other features may, e.g., include, or be performed withrespect to, exactly one X, or a plurality of Xs, in various examples,and that the described subject matter operates regardless of the numberof Xs present.

This document expressly envisions alternatives with respect to each andevery one of the following claims individually, in any of which claimsany such reference refers to each and every one of the items in thecorresponding group of items. Furthermore, in the claims, unlessotherwise explicitly specified, an operation described as being “basedon” a recited item can be performed based on only that item, or based atleast in part on that item. This document expressly envisionsalternatives with respect to each and every one of the following claimsindividually, in any of which claims any “based on” language refers tothe recited item(s), and no other(s). Additionally, in any claim usingthe “comprising” transitional phrase, a recitation of a specific numberof components (e.g., “two Xs”) is not limited to embodiments includingexactly that number of those components, unless expressly specified(e.g., “exactly two Xs”). However, such a claim does describe bothembodiments that include exactly the specified number of thosecomponents and embodiments that include at least the specified number ofthose components.

It should be emphasized that many variations and modifications can bemade to the above-described examples, the elements of which are to beunderstood as being among other acceptable examples. All suchmodifications and variations are intended to be included herein withinthe scope of this disclosure and protected by the following claims.Moreover, in the claims, any reference to a group of items provided by apreceding claim clause is a reference to at least some of the items inthe group of items, unless specifically stated otherwise.

The invention claimed is:
 1. A measurement system, comprising: a flowchamber configured to propagate a sample in a flow stream; one or moreoptical sources configured to irradiate the sample in the flow stream;one or more detector systems each configured to receive resultant lightfrom the sample and, in response, provide a photon signal; wherein eachof the one or more detector systems further configured to process thephoton signal to count photons based on i) single photon pulses and ii)integrating photo-electron charge over a time interval from the photonstream present in the resultant light.
 2. The measurement system ofclaim 1, wherein each of the one or more detector systems includes oneor more semiconductor photomultipliers (SPMs) selected from the groupconsisting of vacuum-tube based photomultiplier tube (PMT), asolid-state photomultiplier tube (SSPMT), a silicon photomultiplier(SiPM), an avalanche photodiode (APD), and a single-photon avalanchediode (SPAD).
 3. The measurement system of claim 2, further comprisingone or more dispersion elements configured to successively provide aplurality of wavelength bands of the resultant light to the one or moreSPMs.
 4. The measurement system of claim 2, further comprisingcollection optics comprising one or more optical fibers configured toconvey the resultant light to the SPMs.
 5. The measurement system ofclaim 1, wherein each of the one or more optical sources is a laser. 6.The measurement system of claim 1, wherein the time interval is lessthan 100 ns.
 7. The measurement system of claim 1, wherein lightintensity from the sample in the flow stream is determined by number ofcounted pulses.
 8. The measurement system of claim 1, wherein each ofthe one or more detector systems is positioned to detect forwardscattered light, side scattered light, transmitted light, emitted lightor a combination thereof.
 9. The measurement system of claim 1, each ofthe one or more detector systems is a detector array comprising aplurality of detectors.
 10. The measurement system of claim 1, each ofthe one or more detector systems includes a spectral element.